I think the best bet so far would be to cut the top of a 10 oz (2.3dl) or bigger oval or kidney shaped hip flask. Cut off the top, add two holes for suspension and of course you stuff it with PSK items. Small enough to be comfortable in your pocket, big enough to be useful (just below your quart pint limit) and with a shape that goes well with suspension from a string or wire. I'll do an asymmetric cut so the opening is slightly lower on one side, this makes filling with snow easier.
That's
my idea exactly.
- Brown - the hip flask split at the top.
- Green - steel band collar, welded or glued to the bottom part (increasing volume and mouth opening a bit), with suspension wire holes, and some means of lid tightening (not shown).
- Blue - gasket glued to the top part (allows to use it as a watertight flask again).
I have 8 oz flask. It's almost 1 cup. But you can find similar one for 12 oz (just google 12 oz hip flask).