When I had my old 06 Cummins with the 6.5' bed I had an ARE shell on top. I looked and looked for a solution, but nothing checked most of the boxes (removable if need be, using all of the space available, etc...).I was a contractor at the time and did most of my own work so it had to work with my tools. I ended up building my own much like big timber above except I used sanded birch plywood for the frame and the deck instead of 2x4s and CDX. It was the best solution for me by far. ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
If doing it again I would get a Decked system. I don't require daily tool storage so a lower height system utilizing drawers makes the most sense. Not too expensive and looks to be very durable.