This is an adapter to fix a Busch+Müller Toplight plus back light to the luggage rack of a newer bike. Actually I do not need the light, only the reflector. The original back light of the bike does not have one, the reflector is fixed underneath the saddle. This is actually not favourable, if a luggage carrier is used, which covers the reflector completely. 
Instead another one needs to be fixed, so I just used an old back light as a reflector. 
The adapter is needed, because the distance of the screw holes of the luggage carrier to the mud guard is so small that the light does not fit in. 

An m5 screw nut is put into the adapter, the adapter is turned round and fastened to the back light using the screws of the light, and the M5 nut facing the light.  Then the adapter can fastened to the luggage rack using an M5 screw. 

There are two versions of the adapter, one for M5, one for M6, because I made the mistake to assume it's M6. So, to avoid another print, I just printed a reduction from an M6 nut to an M5 nut. 
Attached are all files, i.e. FreeCAD, STL, M3F and bgcode.

Because it's outdoor, I used PETG, Prusa Mini+, 0.2 mm layer height, ("quality"), 30% infill, 3 perimeters, and organic supports for the M5 nut hole.