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

Mail Slot Catcher White Indoor Mail Catcher, Durable Oxford Bag for Front Door Garage Office Mail Slot Cover, Space Saving, Self-Adhesive, No Screws Needed,

There is a metal flap on the outside of the wall and nothing on the inside I can even see daylight in the corners of the slot I'm tempted to cover the mail

mail slot cover plate Mail Slot Stainless Steel Satin Color with Spring Flap Fits to Most Doors

mail slot mailbox Letter Box Plates, 10 x 3 Designed for use on exposed doors to prevent wind from forcing rain and snow through the slot Very large opening will