We have a nice brick mail box, with no mail pick up flag. I tried attaching one of those plastic retrofit flags, but refused to fix it again the second time it broke. While looking for possibilities, I saw a magnet that attaches to the front of the box. I can make that. I whipped up a simple graphic, laminated it, and used a section of self adhesive magnet tape on the back.

I was a little apprehensive to see how the mail person reacted, but when I went to check, there was the magnet, set neatly with my incoming mail, and the letter to go out was gone. Phew! And if it ever does go missing, it is easy and cheap to remake. If you want to make your own, print the graphic below on card stock (I printed mine 5” wide), laminate it (I used a heat laminator, but two pieces of clear shipping tape would work), and attach a self adhesive magnet to the back (available at craft or hobby stores).