How to cover this mail slot up so cold air won't come in?

mail slot cover plate   Dan mailbox slot for door

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plate face is 14 wide X 6 H Through the Wall Mail Slot Package includes flap, back plate, and fasteners SKU: 28472 Categories Central Air Conditioner Cover - Round - 34 X 30 by M-D Building Products

accesorios slot Switch Plate Covers · Wine Racks · OUR BRANDS · Log in Countryregion Canada , United States Update countryregion Countryregion Canada  Get another brass mail slot cover and screw it onto the inside end of the slot slot's backing plate with a piece of light monofilament