I use ammo cans for many things. Here is one I use in my truck as a simple lock-box against opportunistic smash-n-grab thiefs. An ammo can with a large hole drilled at the opening end, a piece of electrical conduit with one end smashed (in a vise) and the other end drilled through, a cable for locking bicycle seats, and a padlock. I attach it to the metal seat frame, and tuck it away under the back seat.

This arrangement is actually more heavy-duty and more secure than the official lockboxes you see advertised for cars. I keep an old hand towel in the box to wrap things up in to protect them. Cheap, easy to make, and very functional.

The box is no longer waterproof with that hole drilled in it, but I figure if you have water that deep in the back seat of your truck, well, you have bigger things to worry about!