I made a couple of boards to replace my original ones that, I guess, came with the boat in 1987. I have no idea if there is an accpeted thickness, but I elected to use 9mm Marine Ply purchased from my local wood merchant. A complete sheet provides enough for two boards and the re-inforcing points for any bench supports that came with the original.
I suspect some race enthusiast may suggest you can make them from a thinner material to get a marginal weight advantage, but the 9mm material is sturdy enough, fits the slot running down the CB casing and when covered with a couple of coats of Epoxy are just the ticket.
Hope this helps.