All these suggestions involving ziploc bags I think will get holes worn through them from pocket carry pretty quickly. I'd look for ways to seal the Altoids tin, e.g. with candle wax, or tape around it with clear lacquer over the tape. That assumes that it's only for emergency use, since once you open it, it's no longer waterproof. A narrow piece of large-diameter heat shrink tubing would be preferable to tape, I guess.