I would get a mudflap that goes from one side of your car to the other because any rocks you flick up are going to hit the back of your car including the rear window (Don't ask me how I know). You could thin the epoxy and brush it on, it will soak in and dry overnight. A big advantage is it will fill any gaps making the box waterproof, at the moment you cant wash your boat on that trailer without wrecking the box.