Flag that you put on your mailbox door. When the door opens the flag falls forward to let you know that it has been opened. Super simple, but super handy.
I just used a magnet from Harbor Freight 22x28x10mm.
I also painted the exposed side of the “flag” to match my mailbox. That way it is subdued when closed and really shows up when extended.
https://www.printables.com/model/455507-magnetic-mail-arrival-flag
The author marked this model as their own original creation.