What features would you give it?

Lightweight and indestructible would be good - very similar to Petzl classics over the decades.  I guess the big question for me is always going to be -  with or without drain holes.

I would make it exactly like the red and black Lizard bags which that genius Dave Elliott used to produce. These fit 2 x 6 litre Daren drums perfectly or a 12 litre diving cylinder. And yes, drain holes for wet caving. (No point lifting water unnecessarily and I can't think of any real disadvantage.)

Sounds like a Warmbac Daren Cilau tackle sack, Pitlamp: http://www.warmbac.com/listing_tacklebags.php

Probably the best tackle sack out there, other than when you need something smaller 

The Lizard bag had aluminium D rings at the top rather than fiddly tape loops. It also had a very well designed base, which made it last a long time. The bag you flagged up Pete does look very similar in many ways - but the shoulder strap adjusters look less convenient to adjust.

Actually, if I was in need of more tackle bags at the moment, I think I'd go for one or two of those!

I would have a circular bag with drain holes, reinforced base, high shoulder strap attachment and adjustable straps, d shaped metal hauling loop away, handle on base and side, max diameter 25cm, no more than 30 litres volume.
This is mostly from using a Landjoff 33HW bag.  The base fails first, the bag seems a little too wide and big so you really get one heavy bag to carry when it's full.  The integrated non twist hauling system always gets in the way and the shoulder straps seem to always be under a knee when crawling. 
Having used a few bags, Warmbac, Beast, Daleswear, Petzl, Landjoff, Lyon and even the Lomo, I would say the one I like the most is the Petzl classique, but I'm too cheap to buy one.  The Landjoff has the fabulous addition of a waist belt which makes a big difference on a long walk in, but I would say a rucksack would be even better and if you bought the slightly shorter 28l then perhaps it wouldn't necessary anyway.

Best bag I have ever had is a Swaygo. If somebody could make a version as good and tough in the UK they would do really well. I bought one in the US a few years back. Because of its ergonomic design it can be worn in all but the lowest passages (I can get to Frozen Deep in Resevoir Hole without removing it). The design also allows it to be towed without snagging eg I have pulled one through the Long Crawl in DYO trailing it behind me with no problem. That size carries my Canon G12 and 4 flashes in two boxes. Oh and it is an excellent buoyancy aid having an Ortlieb style closure. It was all I needed to get me across all the lakes in Hundidero Gato a couple of years ago (the inflatable rings we took were useless).

Pitlamp said:

The Lizard bag had aluminium D rings at the top rather than fiddly tape loops.

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I thought the Daren Cilau bag had a metal loop to be honest, but it doesn't appear to from the pictures. The 100m bag is basically the same size but a bit less tough. It does have a stainless D ring at the top though.

While at uni, I found a manufacturer selling bags for tree surgeons that looked a lot like caving bags. I got a sample (which seemed good) and sent dimensions for various size bags and it worked out about half the price of Warmbac and made to measure. Unfortunately the bags lasted less than half as long though, so a false economy in the end...

If anyone is serious about making a new range of caving bags, the key thing will be longevity, then price. If you can't make something that lasts as long or longer than what's out there already for considerably less money, I can't see it taking off.

A well known caving cartoonist mastered the Helium bag in the mid eighties
Whilst a little frustrating for others with a heavier bag, the concept was genius
It would also include a "voice pitch" warning system in case of leaks to warn you that things were about to get harder

My go-to bag is the Petzl 30l bag (because I like to drag too many things around, apparently) with a 45l bag for bigger trips - the material is pretty tough (as long as the contents are padded); the clip-in point is fabric but seems very robust, and the bag keeps water out pretty well (no drain holes for me). The lid does get a bit destroyed in time but that doesn't actually really matter. I am wearing out the bottom of my older 45l bag slowly though - but they can be repaired (boat adhesive glue and PU-coated nylon patches; they aren't PVC). The waist strap on the 45l bag is also great when you can carry it on your back heavily loaded for walking caves or long walk-ins. You can get 200m+ of 9mm in one.

Two things are very annoying about the bag though...
1) for most dragging they are fine, but for dragging when full in rocky passages they always roll over so the off-centre attachment point is at the top and then they tend to dive into every obstruction... it should be possible to construct something to incorporate extra bits of cord or something to make it drag better by running them through the tightening cord holes.
2) (the really annoying one) for me, they wear through at the base of the shoulder straps where they are attached to the bag, and it is a bugger to fix (on the 30l you have to sew onto a patch and glue on, on the 45l it's very hard to fix). You never need to adjust down that far, so if you get some fat tubular webbing, and slide it over the straps and glue in place you can protect the connection from abrasion (you need to cut out a bit of the webbing where the strap attaches to the bag so it is covered).

also the fact that they are very expensive :O
For warm tropical caving though, the 45l bag is great

I have the 15l Petzl, which is nice, but I don't like the built-in donkey, erm, appendage - it's short and very thin. I'd rather have had a robust clip-in loop and use my own. It looks like the 22l bag has the same bit of cord?

Incidentally, all the Petzl bags are made without drain holes, and you can be free of the tyranny of drain holes if you ask someone (such as the sponsors of this forum) nicely. I assume you could also get Warmbac bags without the hole but they might need to be specially ordered or something?

Ladies of some refinement prefer Petzl Classique in my experience.

I really like the AV 25L, 35L and 45L comfort. The TSG have several 25s and 35s and they are our go-to for ropey trips. They seem to have held up to almost weekly use for a few years now. The 45 is very good to put a heavy Peli 1200 in, a pair on 7s or a single 12L cylinder, and carry for long-distance due to the waistband.

I quite like the beast mummy off Tony seddon.
Dragged it all over the place and

cavemanmike said:

I quite like...

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Not a phrase I would ever use in relation to a tackle bag. Tackle B@$t@rd more like!

Wardy's mention of an anti-grav bag sounds more likable. Perhaps somene can invent an anti-grav Tardis bag, that weighs nothing when full, holds 200m of rope and fits in your oversuit pocket? 

The reason I as Is because i have a sewing machine and some cordura and, perhaps stupidly, thought: how hard could it be?
I have made 4 caving bags so far with each one getting better.
Some features I have added:
- Carabiner loop at top (but i like the idea of a metal d ring that some people are suggesting)
- Drain hole in the bottom
- Adjustable shoulder straps
- Carry hand down the back between the shoulder straps

They are all cylindrical. I think the bases need more work. It is really difficult to get the size of the fabric right to sew into it.
You have all made me think about how to reinforce the base....

Just noticed a suggestion of a base carry handle and a waist strap. Is that a popular idea?

JWright said:

I have made 4 caving bags so far with each one getting better.

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You'll have to share some pictures 

Carry handle on the base is really useful for dragging through narrow crawls and also for tipping the water out a bit quicker; definitely worth including if you can.

Likewise metal D-ring is a useful feature. I think I've got quite a few of these knocking about from dead tackle sacks, so PM me if you want a couple posted.

Just a thought, if you are doing this for your own use, why bother with the cylindrical base? If you just sew the base flat (like a beast bag?) when it wears out, you could just re-sew it an inch higher up and keep on going until eventually you end up with a rigging bag... You might have to re-sew the straps each time, but it would be a lot less faff than making or repairing a base for a cylindrical bag.

I believe that the best bag comes equipped with its own porter.

JWright said:

Just noticed a suggestion of a base carry handle and a waist strap. Is that a popular idea?

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base carry handles I find very useful for caving in UK caves with 20-30L tackle sacks. personally only time I've found waist straps helpful is for either caves with long walk ins or with caves where you don't need to remove the bag regularly - mostly in asia. Otherwise it just becomes another strap to get caught on something.

I really like my Warmbac Huatla which is an oval base 35l.  My only experience of cylindrical bags are some club meander bags which are a pain in the ass to pack. The Huatla is arguably a bit more snaggy but if a cave is small enough that the shape of a bag makes that much of a difference - I am unlikely to be in it!

My list of requirements

Tough
Drain holes
A metal ring on top is nice as opposed to just a fabric loop
A nice little pocket on inside for small bits
Shoulder strap adjusters at the top - why oh why are the Huatla ones at the base of the sack - absolute PITA
Clip loop on inside for rigging tackle / crabs etc