Inaccessible voids are a NO-NO!
I don't know if this is actually help for you, RAF. but I will say by way of observation (and experience) that there is absolutely NO excuse for any boatbuilder to include ANY voids that cannot be opened and inspected. As sure as there is water (wait a minute-- there is!) these spaces will get wet and eventually rot. (Yes, even fiberglass 'rots'.) As a boat owner it is your job to find and examine these speces.All over my 1974 H-25 I have cut small openings, installed round deck plates, and made new bunktop lift-outs, all for the purpose of knowing what is in there and being able to do something about it. The one void under the internal fuel tank I am filling with two-part closed-cell foam. There is a drain tube under it connecting the under-seat locker spaces with the main bilge.Under my v-berth are three nice lift-out panels and three separate compartments, but I am installing drain tubes between them to be fitted with rubber stoppers. This way I can drain any water that unitentionally gets in there.NEVER underestimate the threat of having standing (eventually moldering) water in compartments you can't air and inspect.