Boiling container???
Besides hiking on the AT monthly I also get in an overnight paddle trip every 3 to 4 weeks, on my PFD is an attached pack and in it I've got a PSK, a small stainless steel bowl to boil water in, and a candle lantern that when placed under a cagoule can heat you up quickly if you are cold/wet.
I did have a thought though on a vessel to boil water-why not the ground? Dig a hole and place a liner in it-think plastic bag like a large zip lock,,,,put something on the bottom that can hold a very very very hot rock, it may take more than one application of hot rock to boil the water-crazy?
I've seen the really big pieces of bamboo (the ones that are 3-4 inches round) used to make cups for boiling, but in my neck of the woods nothing like that grows.