I recall a story about O'Grady, where after being shot down he found a sponge useful for collecting dew for water. I imagine this would be faster than tying rags to your shoes (as the sponge will hold more, and will also give more water when rung)

It could also be useful for personal hygene. Of course, it can be sterilized periodically through boiling. Vaccum packed, it shouldn't take up much room at all. May not fit into a tin, but might be useful to throw in a belt sized kit or larger.