Holder / case for Xiaomi (V2) Aqara Door sensor and magnet. For the V1 (round) version, see https://www.thingiverse.com/thing:2791484.
You can get it easy out for battery change.
You can stick it to the wall or use some screws.
I printed with ABS. If you print with PLA there can be some space between sensor and case because of the less shrinkage. You can put a piece of paper underneath.