I share your concern Ken, and I was going to apply at least a coat of paint to the plywood but I have no room (kitchen + dining room are the workshop, no garage or shed). Perspex would be too flexible, or much more expensive for a thick enough sheet; I'd also expect harder to cut with an off the shelf el cheapo jigsaw? Acrylic Sheet?: I have no idea what that is.
Please tell me, because the plywood was pretty bad to work with.
Anyway, my scope spends most of its life indoors in a heated dining room. As far as I know, dampness does not easily enter "exterior" plywood, but moisture can build up gradually. This is why I used plywood & not hardwood or god forbid pine. But those who store their scopes in the garage and consider doing the same mod I did should rethink materials and finish.