As a compromise between foil (easy to carry, hard to use) and a metal cup (great to use, not psk sized) could I suggest foil pans - disposable loaf tin sort of thing. Much stronger than foil, and fold up pretty well (you may need to trim the rolled top edge.). I wouldn't want to keep folding and unfolding them, but one that would hold about a pint, carefully folded and flattened will pack small and open up fine.

Just another possible stop point on the size / utility continuum...