Two more ideas for you:

For small portions,cooking in a plastic baby bottle liner can work.

I carry my dehydrated/freeze-dried meals in repurposed mylar snack bags. I find they not only stand up to the boil-and-dump meals but also hold up to having that food heated in the bag, in boiling water. I do usually iron the bottom of the bag in case it has developed a leak when opened the first time.