If you're looking to heat up enough water to sterilize, then a tobacco tin should be good enough, no? Mine measures 3-1/8"W x 4-1/4"L x1"D (78mm x108mm x26mm), and holds 4 oz water. The Altoids type tin is a little smaller but, again, would hold enough for sterilizing.
Someone (fixer27?) mentioned putting hot stones in a container. On one of those survival shows a few weeks back, the "expert" poured water into his canvas boonie-type hat, then placed hot rocks from his fire into it, removing two or three with sticks and replacing them with new stones after 30 seconds or so. To my surprise, the water actually reached boiling point. Lesson learned for me.
Most canvas hats, mil-surplus to Tillys and others, have venting holes that need to be filled with something. But besides that, I imagine anyone of them should be able to do the above. A heavy canvas bag, or even a cotton canvas coat a la Filson, could hold even more water, although they would probably require exponentially more effort to heat up that much more volume too.
Of course, if one takes a spill in a river, goes overboard, etc., I suppose one might well lose the hat, bag and coat. And if you still have your coat and/or backpack/bag, you should have a small metal cup or pot anyway. But the redundancy, and creative ideas for using of "what have you" might be worthwhile anyway.
As per bowieman's father, I grew up carrying a Sierra cup hooked to my belt when hiking or camping, but haven't done so in many years. But keeping a metal cup secured to one's body, with aluminum foil as a backup in a survival tin, might be the best idea if one "absolutely positively" must have something to boil a decent quantity of water.
