QuietFlap Rubber Seal Excluder

If your main complaint is a letterbox that rattles and bangs in the wind as much as it lets draught through, this rubber-sealed design handles that particularly well. It's built to be weatherproof, so it holds up well on exposed front doors that catch a lot of wind and rain. Over several years the rubber will likely need replacing before a bristle brush would, but for the first few years performance is strong.