Coat all the plywood with epoxy, sand, re-coat, sand lightly, then paint it white with PU paint (to keep it cool), then install.

Do the first coat in the evening when its cooling down, with a slow hardener, that way the cooling wood draws the epoxy into the grain. If you do it when its hot, the wood out-gasses, and you don't get a watertight seal 'cos of all the little bubbles that WILL appear.

Make sure the wood is dry before doing it, and that ALL the wood is sealed, edges included.

That's what I would do.