An adapter which allows to mount saddle-post lamps on the rear rack. The holes are 50mm apart, which appears to be the standard for rear rack lights. 

The cylinder has a diameter of 22mm, which is the handlebar standard (all rear lights I had were suitable for the handlebar as well). 

There are two versions, one for thread inserts, and one which has to be tapped. I prefer to tap 3D prints, and I think in this case it's better, since you won't need thread-locking glue, unlike the thread inserts, where you'll need the glue for sure. With the tapped plastic, you just need to be careful not to strip the thread of course, but 1Nm should be possible.