Actually, it wouldn't take much to close it off with a piece of waterproofed plywood. Cut it large enough to overlap the opening by an inch or two around the perimeter, then either bolt it directly to the transom or build some type of bracketing with planks on the inside. You can seal the edge with butyl tape, or some other flexible sealant. Remember, that there is a drain hole in the transom that you must consider... a bilge or drain plug will work well here...Thanks for your response Joe. My guy thought he heard of an easy fix. It's a white plastic (like a cutting board) that is sold at West Marine. It comes in various thicknesses and can be cut with a circular saw. It's supposed to be strong and cost $100+. Has anyone heard of this method? Much appreciated! Mary