your altoids tin is round? or have you seen the new altoids tin yet? im thinking a rubber O ring or a "ranger rick" Rubber band (which is just a 1" piece of bike inner tube) wrapped around the outside of it, you can easily remove it and replace it. try it, wrap the tin with the fat rubber band and submerse it (with the contents removed of course...) for like a night or something. the rubber O-ring might be a little bit more complicated.