My boards are made to the exact factory dimensions on the GMCPaul's website. I used FACIA board, not deck material. I chose this because the thickness was the exact same as factory bed wood. I bought 10' lengths since 8' is too short for a long box. Cut to lenghth, rip to width, one pass on edge with a dado, then one more pass for the deeper channel. Took about two hours to make the set, and I'd never done bed wood.
Just go to the store, and grab a couple samples. Find something that is the right thickness, is wide enough, long enough, and start to make your plan.
