Aluminum foil comes on a ROLL for a reason. Now you know what that reason is.

If you insist on folding it, it will only be good as a dry goods container or a hat.

If you want to take it with you to use for water, you'll have to keep it on something like a straw or a dowel.

To use it to heat water, remove it from the dowel, set it on something roundish (like a rock or your head) and form it gently into a pot shape. Then set it down and scrunch-roll the edge all around so you have something to hold onto that will help to distribute the weight.

How you would boil water in it without having a rack of some kind to set it on is beyond me. Putting hot stones in it is the only other way I can envision doing it. Tongs or a metal spoon would be nice. A dipper would be a good way to transfer the water.

I don't really think anything is going to sit in a PSK and hold water months or years later. Keep a small pot, cup or billy in your bag and you won't have to worry when you've got enough other problems.