I have access to a great solution here in Brazil for PSK water storage. The problem is that this monstrosity is only sold here as far as I know.
Every gas station in Brazil sells five liter bags shaped like a jerry can to stranded mortorists to take GASOLINE back to their car. Now if the concept of putting five liters of GASOLINE into a heavy gauge plastic bag doesn't scare you, you are really juiced up. Nuff said on that.
These bags make awesome water carriers and roll up about the size of a large Bic lighter. They have a spout and four finger holes at the top. I have them stashed around in my various packs and kits. They cost me about $1.50 here.
The next time I go back to the States (I don't know when) I plan to carry back a few hundred and turn myself into the PSK waterbag king.
I like them so much that whenever I run my course I make each pair of guys maintain one full for the three days we're out. So far I've only had one get a hole poked in it climbing up a slope from a creek a guy snagged it on some cut bamboo. They hold up for three days of rough handling and can be fixed with duct tape in a squirting emergency.
Water station in camp. We hang the bags of raw water and keep the filter handy.
Mac
Five liters is enough to live on. Mac
Edited to add: I use the platypus bags as well and they are really tough. I once dropped a 2 liter full into sharp rocks (by accident) and it didn't pop. I like to use an empty as a hammock pillow stuffer or if I'm sleeping on the ground I use a partially filled one as a water liner inside my Ziggy pillow. I like them as reverve water carriers but due to the screw on lid they don't really get flat enough for a PSK.