Anyone successfully use an entry sensor on a metal mailbox?

I tried putting an entry sensor on our new mailbox.  I can get it to work about half the time.  I tried just the bing bong like every other sensor in our house and I tried secret alert.  Both only work once in a while.  I also mostly have an "open" sensor on it.  I tried their suggestion in the help section about putting additional stickies behind it... didn't work. Even went so far as to put both sensors on a piece of plastic and attaching that to the box/door.  

I've seen a couple of old reddit posts where people did it... anyone have any suggestions!?

thanx!