Height above bed rails

I'm picking up my new to me truck camper tomorrow and have a question about how to gain height above the bed rails. My bed interior height is 20", and I believe the "up" height of the TC is 20.5", maybe 21". I've been advised that you want the TC about 1.25 to 1.50" above the rails. I know I could set a piece of appropriate thickness plywood in the bed to get the correct height, but wonder if plywood has too slippery of a surface that might allow the TC to slide a bit. I can do something more permanent when I get home if necessary, but does anyone use plywood?
Any advice or information is greatly appreciated.