I've been using 1L and 2L I.V. bags (emptied of expired saline). Tougher than all get out, but you either have to cut out the access ports or make a rig for filling.
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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