I just ordered a ZSE41 with the intention of using it as a mailbox monitor. Comments welcome.
Zooz 700 Series Z-Wave Plus Open|Close XS Sensor ZSE41 for Door, Window, and Gate Monitoring. Hub Required (Sold Separately). Compatible with SmartThings, Hubitat, and Ring Alarm. Amazon.com
I plan on putting the Z-Wave part on the open-down part of the door so, hopefully, it will be outside of the metal box to transmit. Otherwise, I will need to try another approach.