My dad used to build those like 20 years ago, they don't attach to the truck at all. The lowest part, I used to call the bed, would fit snugly between the two sides and it keeps them in place. The bed part can pull out and go ontop of the little steps sticking out from the seats and it pushes out same way and they don't move. Used it alot growing up. nice way to hide the fishing poles, guns, beer, =-) we made a small cooler in one back corner. and had cup holders up by the cab and in the back too..
we never mounted them to the sides at all though. never needed to....
If that were mine, I would take the bed part and re-foam it, looks a little flat.
12 F150 Lariat 5.0 4x4 Crew
03 Edge 4.0 -Next door
Y2K TJ Sahara... Lifted, locked, loaded
08 Scion tC