I agree with you about the virtues and drawbacks of Tupperware, although, theoretically, you could do stone boiling in a plastic container. I have never tried this technique, but it is mentioned in all the survival manuals, so it must be good, right? (In theory, theory and practice are the same, but in practice,they are different - according to yogi Berra).

I solve the metal container problem by slipping a cup onto my canteen, where it takes up minimal space, ready to make me a nice cup of tea.