Don't know where you ended up with this project...but thought I'd offer the following for anyone with similar objectives.
I found that trying to rig anything with the standard mailbox was going to be fraught with issues. Changing out to a more secure mailbox like below (available at Lowes and elsewhere) opened up a lot of options. https://www.mailboss.com/
I have a Zigbee contact sensor (Visonic MCT340E) stuck to the inside wall of the box with the magnet on the underside of the pivoting door. Nothing can be seen from the exterior, door open or not.
Unbelievable to me this has worked THROUGH the metal side of the box to a repeater ~50' away. Furthermore, the dang box heats up to 120F in the sun. I think I've had to change the battery once a year!
Expanded information per community request: View is looking up into the unlocked bottom door of the box; sensor on left wall, magnet on bottom of the pivoting mail drop door above (slightly opened for this picture to show magnet separation from sensor). Sun had heated the metal box so hot it hurt to hold that drop door open for this picture. I'm still amazed this sensor survives these extremes. Been at least two years now.
