Great idea with the instructions printed (in large print) on the bag. I'd like a pleated bottom so that it stands up, and if it could stand up when empty, that'd be even better. Pleated sides would keep the zip lock top smaller so that folding doesn't ruin it. In short, a filled size of about 3 1/2" diameter and 8" tall (I think that's about right for a liter) with a 2 1/2" or 2 3/4" zip lock top.

As long as we're wishing, a little tube - flat molded, same plastic as the bag, 3 or 4 inches in length and made water tight by folding back on itself. Like pinching a garden hose. Maybe stays folded using a Velcro dot. Out of the top corner of the bag. It would make drinking the finished/purified water much easier.

Throw a couple of MicroPur foil packs in the bag, fold it up, and you're good to go.

BTW, I took a pair of scissors and rounded off the corners of all my MicroPur foil packs. Those foil corners are pretty stiff and sharp, maybe sharp enough to cut/poke through my PSK. No sense in taking a chance.