Switchbot Lock and Lock Ultra come with a magnet you're supposed to stick to your door frame with a double-sided tape. This magnet allows the lock to detect if the door is closed or not. 

My magnet kept dropping after a few weeks which resulted in me trying to find it on the floor / stuck in my vacuum / in my dog's cage. The door frame I have is an old wooden frame, far from being a good surface to stick a 2-sided tape to. So I printed this simple cage that I put the magnet in, screwed it to the frame and the problem is solved. This is why I love 3d printing. 

Prints with small supports, in 21 minutes with PLA on my A1 mini.