Re: Questions on replacing plywood under sunpad/engine cover
I recommend that you at least put a thick coat of durable latex paint over the replacement wood on the top. That will do a bare adaquate job of water resisting if you fear fiberglassing.
My recommendation is you do a single layer of chopped strand mat fiberglass over it on the top and the sides of the wood that will replace it.
Having the chopped strand mat cover the side edges of the plywood is not a big deal. However, sealing the edges with sufficient polyester resin and coat a very small portion of the underside from the edge. 1/2 is perfectly fine.
However, do NOT go overboard on too much csm or resin, otherwise it will be REALLY heavy and the shocks holding the board up will struggle or fail. Fiberglass it good enough to get 1 layer down and properly coated.
If you drill or add screws into the board, do that first and get it well fitted before fiberglassing. You do not want any holes that are not sealed, nor do you want any exposed holes on the top.
Re-covering it and fitting it is a little bit more involved than rebuilding a removable deck gas tank cover for a boat floor, but very close to the same concepts for making it sturdy and shedding water well.