Here’s the pics of my back bulkhead. Each sheet of steel is 3ft wide by 4 ft tall. They are backed up with 1/2 inch (IIRC) thick plywood. All around the top of the gun box is 2x12” boards. Those serve two functions. 1st, safety. If you are riding in the back and loose your balance, it’s better to smack into wood than 1/16 inch steel. The other function is proving support for the walls. Without support, the outer bulkheads kinda wobble back and forth. With the 2x12’s, the walls are pretty solid.
Now to the aft bulkhead. Starting on the drivers side, install a sheet metal/plywood panel across the back. You’ll need some support to hold it in place. Then go to the passenger side and do the same thing. Finally, install a third panel in the very center of the aft bulkhead. It’s important to get it as close to center as possible. Attach the left and center panels and brace up the center panel with 2x12 at the top and a section of angle iron at the bottom. You don’t want a section of 2x12 on the right hand panel. Add one more section of 2x12 vertically on the right hand wall. You need at least two fence type hinges attached to that vertical 2x12 and the top section. Two seemed best. As long as everything balances out, it’ll be solid once you close it up. Last details: figure out a locking mechanism so you can secure the door while you are in the box.
that’s the way I did it. It was much simpler than writing it all out. You’re indicating you’ll have secondary bulkheads internally. The aft bulkhead is relatively easy to change where you want the door. One of the pics shows the two fold up seats I got from a guy who wrecked his boat. I need to mount them, just got to figure out the details. Pics below: