To really seal it, get some marine epoxy (West system or other), hardener, and thinner. Thin the epoxy to almost water consistency and saturate the plywood on all sides and edges (including hole edges). You wont have to put any cloth or mat on it, just the epoxy. If you are going to store these outside, it would be worth giving them a coat of compatible paint as epoxy is somewhat sensitive to UV.

Regards,
Bill