You could use any lamp with a "cold" light to make the room more cold, and use candles, like in Snape's class. Also you can put a cauldron with dyed water or fruits punch, and put a smoke bomb around so it seems like Seamus has been there :P Also you can put black tape or staple black streamers along the wall to make it look like the dungeon's wall. If there's something in bright coloursm remove it. Also put some jars with fake sugar eyes or brains. Hope it helps!