I found the aluminum box at a store on Sanibel Island Florida and I don't know the name of the store - sorry I was just a drunk tourist If you search around the internet there are several plastic and metal soap boxes without hinges for sale. Amazon has several. Ebay has quite a few too. Aluminum boxes are harder to find but they are out there.

Check out this box from Amazon: http://www.amazon.com/Goody-Lifestyle-6-Pack-Assorted-Colors/dp/B004RHT512 This might be snug enough that would not have to add a gasket of silicone or a rubber o-ring. It may be waterproof enough just by the friction of the lid.

The key to all of this though is that a plastic or metal travel box for a bar of soap is the ticket. It is almost exactly the same size as an altoids tin.

For the poster who asked why not just put an o-ring on an Altoids tin - the problem with that is the hinge. An o-ring on the lid would not address water getting in at the hinge. The solution is to have a box with a lid that is not hinged.