I took an old platypus bottle and cut the bottom 3rd off. (This is I suppose similar to the suggestion of re-purposing a Sawyer bag....) Folds to nothing, weighs nothing, and is in fact so much nothing that I've forgotten I even had it a couple of times. Incidentally, I cut it to hold just shy of 8oz, so I'm also able to use it as a measuring cup, if and when that is ever needed.