You don't have to build something to fill the space.
My Tartan 30 has a large hanging locker - more space than the Admiral gives me in our bedroom closet. But useless - I'm going to hang my foulies so they don't wrinkle? At most, I need a hook to hank a jacket and a pair of slacks. Emptied the locker and build in shelves for storing stuff in plastic baskets - buy the baskets first! Got it done, and still had room for the hook and the jacket/slacks. I think the only reason they give you hanging lockers is that they're low cost to build.
I'm expecting the design where the main cabin is really a walk-in closet - no need to build seating. GO to your room!