2x6's. I used black painted plywood to keep what was carried underneath concealed from the casual eye. Kept the back seat clear and available for mid day naps on long driving days.
The space below the plywood and bed floor was the same padding used by backpackers then that was topped by the (clean) 4x6 Aramark hall carpet that was to be discarded when my company changed its name. Keeps things from sliding around and my dog likes it too. When I go to the range that becomes a mat when I go prone to shoot 500-1000 yards.