This post is in reply to the original You’ve got mail… and the Thinking outside the metal (steel) mailbox 2.
Below in pictures is an externally placed contact sensor for monitoring when the mailbox door has been opened. This configuration works with any mailbox material. The sending unit is sheltered from the rain in an open bottom case with a spring latch
to hold it in place. Only a piece of tape is requires to retrieve it to replace the battery. It is easy to construct, and even easier if you have a 3D printer. With a 3D printer, no cutting, and only three parts to glue together. This one was built for a neighbor.
I’m glad you like it raym64. I did discover a shortcoming. When running my fingers along the bottom of the case attached to the side of the mailbox, the latch spring can be flexed and the sending unit subsequently released. Future cases will have longer sides and faces relative to the latch spring.
For the unit already made, the latch spring was shortened and thus recessed and not accessible to the casual touch. It is very good now.
See the photos below.