For a water container, think about an oven bag (the kind you just cooked a turkey in). I soon plan to fill mine with varying quantities of water, so I can get volume marking dimensions (1 cup, 250 mL, etc etc).

Once I figure out a "scientific" method of filling, securing the top can easily be done with a length of twine, rubber band, or binder clip.

The bags are very tough, take up less space than breast milk bags (although I love my IV fluid bags...too bulky), hold more.
Member of the toughest, meanest, deadliest, most unrelenting -- and ablest -- form of life in this section of space, a critter that can be killed but can't be tamed. --Robert A. Heinlein