Never tried these myself but they look like an interesting concept....
http://www.swaygogear.com

I'm always a bit dubious of the waterproofing layer being the same as the protection-from-cave layer, but it does look like a nice design and I'd be interested to have a play with one if I found one. Not interested enough to buy one though I don't think.

All down to comparative cost with other products. Mr O' Doc loves his.

They are extremely tough. I appreciate that he makes a pretty small one. I don't know how space efficient the design is, i reckon it depends on what you're packing. I like mine, though I've been ridiculed by other cavers for having too small of a pack. On a long survey trip my wife asked for some painkiller, which I had neglected to bring. We got some from another caver and for the rest of the trip had to endure constant jokes about the lack of space in my pack for an aspirin tablet.

If anyone is interested in supply chain issues for niche products, in the podcast below the Swago founder Scott McCrae describes at length his issues in getting a suitable material to make the bags.

https://www.podomatic.com/podcasts/cavingpodcast/episodes/2017-08-01T07_39_23-07_00

And a recent follow-up
https://www.podomatic.com/podcasts/cavingpodcast/episodes/2018-08-01T06_01_02-07_00