My boats don’t see salt water and they are kept well covered and ventilated when they are not in use, so I use exterior grade or cabinet grade plywood for transoms (I have done three). If you can find marine plywood, all the better. I typically finish them in varnish, inside and out, knowing that it is difficult to keep water from getting into the wood through fastener holes but again, these boats don’t spend extensive time in the water or exposed to the atmosphere. One thing: you will have to measure the thickness of the original inside transom piece and make the new piece the same thickness. It has to fit that aluminum C-shaped cap. You may have to glue two thicknesses of plywood together. Unless the boat will be exposed for long periods to salt water, use stainless steel or galvanized fasteners. They look nice and will not rust.
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