Let me elaborate: The modified hip flask is not for water carrying - but melting snow and boiling water. It would boil about 2 dl (6-7 oz), which isn't very much - but it is small enough to stay on my person when my pack doesn't and far more reliable than any collapsible aluminium foil trick.

If I have my pack I have at least one water bottle and at least one cocking pot (or something to that effect). And - water availability is usually not a problem in my part of the woods.

ALEX: That is a very neat idea. I can't weld (though I know someone who does), and anyway 2 dl / 6-7 ounces is just to feeble amount to worry about carrying anyway - gulp it down and be happy For carrying more substantial amounts there's always the condome trick or the breast milk freezer bag trick.

One scenario that is important for me is melting snow. Adding snow to a narrow slot is painful and tedious. For this purpose I suggest that the flask opening has a shallow "U"-shape at one side (about 1-1.5 inch deep). This wider opening should make stuffing snow in there much easier, and you could also use the hip flask to scoop up snow directly.