Rotten plasterboard. Yeah, they will insist on replacing the whole panel of the same below building standard shit. If you want do it yourself, but better, pull the nails, get a panel of lets say made of 1×1s spaced by 1×2s creating a vertical grill that fits the space you need replaced (inside a 1×2 'picture-frame') and nail each together as you add them. Then you can file a pattern of groves and notches in the outfacing egdes of the 1×2s to make it look old and antique picture frame. I recall a diy guy who blow-torched his untreated timbers, and then nailed them together. I recall the doctor who tardis had a dark red-brown varnished wood room with routered edge circles for an antique style if thats your thing, but something that is ultimately nailed together as a timber panel that can be attatched to the wall behind it by screws is likely your thing.

Or something quick- a painted polycarbonate panel that fits the whole section attatched by hinges and a magnet pair on the inside.