I like the idea of the door incorporated into the cushion, provided of course you have some pretty solid latches. On Raven our settee backs double as cockpit cushions. (Very clever of Mr Van de Stadt's menials to design that into her). Plywood insert would make them harder to move about.
Me I think pilot berths are a waste of space in small boats. Give me the storage space and I'll make do with a lee cloth on the settees.
I reckon the idea of two of three openings, hinged at the bottom and with good solid latches is the way to go, rather than just the one.
“Give a man a fire and he's warm for a day, but set fire to him and he's warm for the rest of his life.” Terry Pratchett