These brackets allow you to cut a hole in your door and door jam and recess your sensors without them falling in and allowing you to pull them out with easy to change batteries. You will need a few tools to cut the holes. I would recommend a router but a drill with a 3/8 bit will do the trick. To get a really clean looking finish take a sharp chisel and cut down 1mm into the wood so the bracket also fits flat. You will also have to find some small screws that will fit nicely without sticking out too far as to hit the door. If screws are not an option some white tape or even scotch tape will probably do the trick. If done correctly no one will even notice they are there.
Install video: https://youtu.be/Dy4T3eIdc5o
NOTE: There is a newer version that makes installing much easier.