My Altoids tin could hold 5.5+ cubic inches of water (measured to the brim), about 3 oz, a little over a third of a cup.
The British soap dish can hold slightly over 13 cu. in. of water (7.2 oz), more than twice as much, .9 cup. Not bad, actually.
I know that an empty pot can warp over heat, but would a container like the soap dish really warp as long as it had water in it? You can make a cooking pot out of some flexible bark (aspen, birch) and put it right in the fire, and it won't burn as long as the flames don't get higher than the water level in the pot.
Any thoughts?
Sue