Since I could not find a model for keys of the "BKS detect" brand common in germany, I made my own very basic version. 

The model can be printed in basic PLA without supports key slot pointing down (you might need to rotate the model in your slicer). The key simply slides in from the bottom (V3) or top (V2).

The cover is loose by design but won't come off if you attach it to a key ring but it will slide a bit up and down when pushing the key into a lock.

V2: This can cause the ring to break if you push too hard or the layer adhesion of the print is not strong enough because the ring is quite slim

V3: way more sturdy but also harder to get the keyring on. Mine is still doing fine after 3 years

Also fits Börkey BO-1900