Hello People and @davsav .
Regarding placing a contact sensor under a mailbox.
I did not have much side skirt protection under my mailbox. More room for the sensor was created by rabbeting out some of the wood adaptor plate between the mailbox and the support post. A piece of 1/8" PVC was used to provide a support shelf for the sensor. The sensor is well shielded and easy to get at. No special sealing was used. It was installed Jan 2020 an.d has worked without fail
Picture 3 of Thinking outside the metal (steel) maillbox - 2 shows how this looks.
Victor Maletic.