For composite and upvc doors we would recommend using our door letterbox which is supplied as a full kit and contains a tunnel and brushes to reduce drafts and insulate the door. The sleeve inside the letterbox also protects the door from moisture and masks the aperture cutting. Overall Size: 315mm x 92mm Opening Size: 249mm x 41mm Fixing Centres: 283mm Designed to match our other polished nickel front door furniture to create a feature entrance door. Maximum door thickness: 72mm.Minimum door thickness: 42mm. Supplied with matching SS bolt through fixings.