I wouldn't use cutting board, or any other plastic, for a backer plate. Plywood is easy to work and lasts a pretty long time.
Solid stock is not a good idea if you are considering using wood. The cross lamination of the grain in plywood is preferred, it won't split and doesn't have internal tension like a piece of oak. I use 3/4" marine ply for through-hulls and I soak them in Smith's epoxy at least twice. I wouldn't epoxy to the hull, as the washer will be removed when the thru-hull is next replaced. Hopefully after I don't own the boat anymore.
Sailing a '74 Challenger 40' Ketch rig out of San Francisco