Thanks guys. Using the stake pockets would be good too, but I like the tie downs I have installed in them, so I sat it on top of the rails with vertical posts inside the bed. The wood sits against the posts keeping the top from tipping outward. The turnbuckles keep it in place, but I had to drive a screw into each one to stop it from loosening up from driving vibrations.
It's just 2 sheets of 7/16" Osb and 2x4s. Ripped to about 18" to be level with the cab top.