I played with the idea a while ago for a kayak kit. Not sure what I was thinking when I look back,as common sense will(should)have told me otherwise, but to make a long story short you won't really be able to make an altiods tin "waterproof". water resistant sure, but either accept the fact that water will make its way in eventually depending on how bad the tin is exposed to water or drop it in a ziplock bag or something similar as others have mentioned. The fact these tins have holes where the hinges are will make it harder.

I have seen soap boxes that snap closed and fit the dimentions you are talking about(ish) They are really inexpensive and may offer a useful solution.