I'm not quite sure what is your skill level and what other tools you may have. That entire back section can be built with a drill and a rivet gun. Buy some sheet aluminum, some top hat for the structure, four hinges and a lock. Maybe look for a junker with swing doors, cutoff the whole back panel and graft to your truck with rivets.
I can't post pictures yet, but we took out the door and grafted a steel box that houses our propane, water heater, exhaust vent, and electrical in dedicated compartments. We just bolted on to the door frame and put some flashing on the outside edge.