I made a mail arrival indicator flag that lets you know whenever the door has been opened.

It mounts to the bottom of the mailbox and has a slider that pushes the flag down as the door opens. The flag and mounting base are print in place, but the slider must be carefully attached to the mounting base. There is optional M2x30mm screw for tensioning the flag if it wears out over time. There are spots for 15x2.75mm and 12x2.75mm magnets to help hold the indicator in position while gluing in place. I recommend gluing as the magnets will not prevent the flag from sliding.

It is specific to the Architectural Mailboxes brand with the decorative plastic shroud on the front and back.

I printed in Inland ASA, .12 layer height, 6 walls, 99% gyroid infill, with normal supports for slider and the magnet holes, with support blockers everywhere else.