Most tins will require a seal of quality tape regardless of design. Happily this gives us a useable quantity for field repairs. BCB of the U.K. markets a tin with a gasket larger than an Altoids but still pocketable. There are some US distributors. I'll have to find them later for you. You can also 'think outside the box' and use round tins. Usually they are twist on and much more waterproof. Look at shoe polish, saddlesoap, candy tins. It's also a good idea to wipe down your kit with a soft tissue. Don't forget fingers leave body salts and moisture.