Create a inexpensive lock box out of an old surplus ammunition canister. Most ammunition canisters used by the military are made of thick steel with a lever closing mechanism. They are generally waterproof and have removable lids. This makes them ideal for storing a wide variety of equipment that are sensitive to moisture.
Step 1: Selecting an ammo can
The best place to buy these is at your local military surplus store, or at a local flea market. They can cost anywhere from $5 to $50 depending on size and condition. When selecting an ammo can pay attention to the holes located in the closing lever. The one I used in this instructable happens to be shaped perfect for the hasp to pass through. If you can't find one like this in the size you want, you can always cut a bigger hole.
Make sure that the rubber seal is still located in the lid if you want it to be waterproof.