The first thing that needs to be done is painting the inside black. This prevents stray light from screwing up the pictures. Just tape off the outside and spray the inside with black spray paint. Then stain and coat with a lacquer or urethane coating of your choice. I like Minwax Helmsman Spar Urethane.
Once dry all the corners inside the box need electrical tape on them to make sure they are light tight. Take a small piece of aluminum and epoxy it inside the box over the hexagonal nut hole. Add some more epoxy to the hexagon hole, and insert the nut.