I approach it from another direction- how far can you strip your gear and NOT have a pot or reasonable facsimile to boil water in. I do not mean folded up and soon to be leaking aluminum foil anything.

This is the reason I like the space saver cups and canteen cups- if you a water bottle or canteen, you should have a cup. Given how well they fit in most of the carriers, there isn't much excuse to not have one. As I've said, you can tuck a coffee can into any place you can put a space saver cup, so money isn't a very good excuse. If you are utterly wedded to carrier that is so tight it can't hold the cup, then switch to a metal bottle on the same form factor, like the Goyets, which fit anything that will hold a Nalgene, no mater how tight.

The only reason to NOT have a pot or cup or a metal water bottle (I find the Goyet's superior in fit and function to the Kleen Kanteen, IMHO) is if circumstances leave you with only what was in a secured pocket or dog leashed to your person, such as mightbe the case if you are crossing a river and a current grabbed you and now that you are out of the river all you can do is wave bye-bye to your pack as it goes down river.

Alternatively, you can use a box like you said. The old French mess tin, with the middle piece removed, is a big PSK, but it will fit in a jacket pocket or into the pocket of most hydro packs. Slipped into a spare watch cap and a pair of gallon freezer bags, it is a good fit in a 6x3x7 pouch. There are also rigid mini loaf pans the size of the foil ones that won't leak and tear. Shouldn't be too hard to make a lid out of flashing or heavy foil, and slip that into a jacket pocket or into a belt pouch.

But if what you are thinking is more heavy duty altoids-sized tin, why bother? It isn't enough water to stress on honestly. I'd rather have chlorine tabs and a turkey roasting bag than to boil water less than a quarter cup at a time.