I had to space my camper up a bit as well, I wager you'll want a good 2" or so of clearance between the cab over and the roof to account for the camper wiggling around.
On mine, I ripped some dimensional lumber to the desired height, screwed it to the camper bottom around the perimeter, placed a a few cross members across the width to tie it together, then filled the voids with 2" thick foam sheeting before screwing a sheet of exterior grade plywood to trap it all in. It's been rock solid over the past 10 months. Sure beats the couple of 2x6s the PO put on the bottom to try and achieve the same thing.