I think this post will help address some of the comments I saw in this thread.
You may want to look into this way of hardening-up a contact sensor inside a plastic mailbox. See the pictures below. This installation method may be reversed to be on the outside of the mailbox. In the inside installation, the pieces of plastic were cut to shape. Those of you with access to 3D printers could, of course, print what you need.
With a plastic mailbox, four screws near the corners of the plastic holder for the sending unit would spread the impact forces. To change the battery, use a key to push the unit out the back and into your hand.