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

Mail Slot Catcher for Door Mail Slot, Black Indoor Mail Slot Cover interior mail slot cover Security guards are metal or plastic devices installed on the interior side of the door, covering the mail slot opening They provide a physical interior mail slot catcher Installation instructions for an Elite Model DraftDodger™ Insulated Mail Slot

689 slot The interior and exterior sides of the mail slots have flaps The exterior flap on the mailslot is spring-loaded and the interior flap closes via gravity Use