I'm a big fan of:
http://www.bepreparedtosurvive.com/Aqua-Pouch%20-%201%20Liter%20Water%20Storage%20Container.htm

I have like 6 of them. I bought their Aqua Plus Purification Kit as well: http://www.bepreparedtosurvive.com/The%20Aqua-Pouch%20Plus%20Water%20Purification%20Kit.htm
(IMO its a good deal for the bag and tabs combo).

I have one of them folded twice - top to bottom, then left to right with zipper on outside (directions from the guy from the site), and it fits in my psk perfectly.
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"One should not increase, beyond what is necessary, the number of entities required to explain anything"
William of Ockham (1285-1349)