DraughtStop Internal Cover Box

This works differently to the brush and flap styles — it's a small enclosed box that fits over the letterbox opening on the inside of the door, so post drops into a semi-sealed compartment rather than straight through into your hallway. Draught blocking is excellent since there's no brush or flap gap to speak of, air simply has nowhere to get through. It's the bulkiest and most visible option here, so it suits people less concerned with a completely invisible fix and more concerned with maximum performance.