Add security to your post mount Lewiston Mailbox System with a locking insert that mounts inside your mailbox. Locking insert is made of galvanized steel and features a tough powder coat finish keeping mail secure from unwanted hands. Incoming mail slot opening: W 6 3/8″ x H 1″. Locking door opening: H 5″ x W 6 3/8″. A lock and keys are included.