Just use some 2 inch flat steel strips of 8 to 10 gauge and weld over the top of the bed rail for strength. That lip will fold flat real easy with a body hammer and a dolly under it for support.

I am going to use the left over top part for my lower back wall as well. Then get some flat 16 gauge to make a double panel upper section, then cut a hole in it for the window. IF you run a "U" lip with a bead roller along the edge you'll get the offset you'll need for the lip on the back where the door trim went. Then it wil all be flush on the back wall. And, have enough reinforcement to keep the oil can effect out of the cab. Heck, a little spray foam between the wall panels and it would insulate it as well.

Too bad your not closer, we could have a ball getting this thing going.

Later,
Curtis