On my 64 Starchief - 2 pieces of 3/4" marine grade laminated together and coated with epoxy. Used wax paper on the outside epoxy coats to get them smooth and keep dust off. A light sanding when dry and use plenty of marine grade silicone sealant on the stainless steel bolts and screws. Make sure your fasteners are 316 stainless not 304 grade. Take your time and get it spotless inside the transom slot before reinstalling... I used Rustoleum self etching spray primer on the inside parts of the aluminum I could reach.
