If your transom is similar to what my ol 12' aluminum boat has, you can scrounge up a piece of 2" doug fir (1 3/4" thick). Put on a sufficient amount of Thompson's Water Sealer. (Let it Dry). Put on sufficient amount of Marine Spar Varnish.
If your talking about the boat bench seat, maybe some home improvement store will sell you a half sheet of lumber core plywood. Or maybe use some 1"x pine or doug fir. Then treat it with the water sealer & Marine Spar varnish.
Best of Luck on your project, Bro.