Cut a strip of paper measuring 4"x 12". These dimensions are specifically for the $1 mailboxes from Target. If you have a different one, you may need a different size piece of paper.
Hold your paper up against the mailbox so that one of the ends is about halfway across the bottom. Now smoosh it really good up against the base of the flag to make an impression.
(Hello, I have man hands!)
Punch out the circle and cut a slit from the edge of the paper into the hole.
Trace the lid twice with a fine tip pencil. These will be the front and back covers to both lids. Only one lid actually comes off. Don't try and pry the other lid off. It may result in the whole mailbox falling apart...not that I did that or anything. Just a random thought.