IMHO a solid piece of plywood is asking for trouble. It will be difficult to keep moisture out of the plywood unless the truck is inside 100%. The original design of planks separated by the rub strip bolts allowed air to circulate on the sides of the planks which I believed slowed the rotting process. Humble belief from wet western Oregon.
Granny: Plywood will hold moisture warp and rot and look terrible in a short amount of time. Use "Kube's" suggestion.
LOOK HERE: https://www.bing.com/search?q=how+t...dIBCTI3NzQ1ajBqNKgCCLACAQ&FORM=ANAB01&PC=HCTS
Or like Ford did it...
pressure treated- but only if stored inside.