I've been using gerber babymilk bags in my PSK for water storage. They hold up well, but they store significantly less than a liter of water, and most water purification tablets purify a liter at a time. If I have to, I'll cut the tablets into smaller pieces, but I'd rather not if I can help it-

I'm looking for a durable one liter water storage bag to fold up and store in a tobacco tin sized PSK. I've seen some storage bags that have a plastic cap, but these don't fold flat-

I do have a "just-in-case emergency water bag" that came in an automobile kit, it has one of those plastic locks that can be seen on bread (not the wire ties). This looks like it holds quite a bit more than one liter though (maybe 1.5 or so, it doesn't say anywhere).

Any recommendations?