Damp Rid is a good product. It's basically dessicant crystals that absorb moisture in an enclosed space. Follow the instructions and you will find yourself changing out the Damp Rid containers as they suck the moisture out of the air, the containers will have water in them.
Having said all of that it may be cheaper to do as others have suggested and put some air vents in the metal box. I wonder if an air vent on each end with a wind turbine mounted on top would be pretty effective?