I had the need to waterproof a glass reed switch (like this https://s.click.aliexpress.com/e/_c2Q1aXir) to use in my mail box notifier so I made this small hollow box which allowed me to stick the reed switch inside and fill with hot glue. I used double sided sticky tape to secure it in place but it has 2 holes so you can use screws instead. This can also be used for the magnet as long as it fits.

I apologize for how dirty it is in the photos but that just proves that it has been working great so far as it has been there for probably a decade 😅