Re: aluminum lund transom
I used 2 pieces of 3/4 plywood glued together with Gorilla glue, it is waterproof.
I started with marine grade plywood but you aren't likely going to find that so go with exterior grade, once your peices are cut out and the glue is dry I would paint them with a basement waterproofer type product,Dri-lock is one kind I have seen do the front back and sides then bolt it in.
Thompsons is good for a deck on your house but I wouldn't use it on a transom board.
Use stainless hardware if at all possible and when you cut the boards start big and trim to fit.
Good luck