Good point. My immediate thought was that I would salvage whatever discarded can I could find for the job, as these tend to litter the British countryside in profusion, but if caught out without my main kit in the middle of nowheresville where there was no litter, I guess it would be a case of having to try boiling water in a leaf or something. I have done this many years ago, but would not want to have to boil all my drinking water this way. Not only would it mean using a hell of a lot of leaves (they tend not to hold much), but you would have to make certain that the leaves you were using were non toxic to avoid contaminating your water.
You might think that a depression in a rock would do the job. Adding hot stones from the fire to the water until it boiled. This would be fine IF you could actually find a rock that would hold water in the first place, and you would need to have a lot of hot rocks to add to it as boiling water for purification requires the water to boil for some time,n ot simply get to boiling then cool again. Lets face it, when did you ever find just the right rock or stone right when you really needed it?
I once saw (But cannot find in the shops anywhere) some extra strong aluminium type foil bags that would be great for this job as well as the "cook it in the bag" that they are sold for. Adding one or two of these, folded up small, to your belt kit would make a good deal of sense. If only I knew where I could get some....