For the last week I've been making the frame for the lid, today I test-fitted the pieces loose, and cut a slice off the top of the front bulkhead between the walls to allow the front lid frame to sit in front of the bulkhead and drop down to the proper height. The edges of the lid are cut to a matching angle with the tops of the side boxes, it's roughly 35-40 degrees. I bought heavy-duty stainless hinges, that will be mounted to the front of the bulkhead and the underside of the front lid frame.
I cut the frames down from three 8' 2x6s on my table saw to ride on top of the side boxes and drop down between the walls, and make an inset space for the 1/2" birch ply lid. I don't have a router, or a dado set, so it was make a cut, slide the board over a blade width, make another cut... Took hours. And doing it that way made for a very rough surface where they ride on the side box tops, as no 2x6 is ever perfectly straight, so the cuts made grooves. I had to chisel off the thin flaps that were left between some of the cuts, then surform-plane the surface to smooth out the grooves made by the blade. I haven't cut the side frames to length yet, but I'm thinking the rear edge of the lid will be even with the rear edge of the bumper to help with the turbulence behind the tailgate.
I used pocket screws to attach the walls to the bulkhead, and put the first couple of carriage bolts in the back of the side boxes through the bed rails, so I could drive it with everything in place. I didn't put every screw in yet, because I still have to take it all apart to make the side drawers. I painted the whole thing with silver grey porch paint on every inner surface, but the outside will be stained to match the truck, then clear-coated.