How about a different approach? Instead of a waterproof tin ensure the contents are waterproof or can stand immersion. Stainless bits and pieces, tinder sealed in a bag or waxed (isn't waterproof tinder a good thing?) and so on. Not too difficult.

Drill holes in the tin so it may drain and ventilate easily so if it does get immersed, which is probably not too often, the contents will dry out quickly.