Thanks very much to JWright for the bag (picture 2/3 from his post above). It's been put through its paces, but after a quick scrub in the bucket (after I'd broken the ice off) it's looking pretty much like new!
I would echo John's advice regarding the webbing for the side handle. I think that this does most of the work when the going is tough, so it's worth putting in the extra effort to strengthen this.
Also, for the benefit of anyone else making their own bag, make sure to leave a bit of slack for the cord through the eyelets, or it has a tendency to jam up when muddy...