Go up to Pep Boys or whatever store is around you and pick up some fiberglass resin. Cut a gallon jug in half and pour some in there. Pour some hardener in it and mix it with a paint stick. Apply it to the box with a paint roller. Let it dry and you're good to go. It will stink up your truck for a few weeks though haha.What's the cheapest way to water proof Mdf??