Large Magnetic Mailbox Covers sorted to make it easier for you to pick the right design for a holiday, season, occasion, theme or vendor. Flagsrus offers high quality, large magnetic mailbox covers from Magnet Works (Studio M). Add a splash of color with a vinyl weather resistant mailbox cover! All covers feature a slot for the postal carrier alert flag. These covers attach to the mailbox with magnets. The large size fits 8" x 21" mailboxes. No worry if you have a plastic or aluminium box, see the Adapter Kit.