I'd probably try using clothes. Maybe make a fire, put a "dry" rock in the coals, and let it heat up. While I'm waiting, I'll try to make a container out of the sturdiest cloth I have on me, fill it up with water, and set it on the rock. I'd hope the rock would not burn the wet cloth, and would get the water up to temp. I don't know if it would work, but I'd have to try something.