The holder that is mounted on the bike has a rail in which the back light is slided in. Unfortunately, parts of that rail broke due to fatigue of material.

This thing restores the ability to securely fix the light onto the holder.

Update 2019-03-20: On installation I noticed that the frame mounting point interferes with the back side. Have updated the model which hopefully will fix the problem (not tested as I simply modified my already printed object).