Bathroom Rail Radiator Towel Hook for rails with 20mm diameter and 20mm distance in between.
There is also a protector clip available (without a hook) if your radiator is behind a door and the door handle hits the radiator.
Printing recommendation:
Use PETG to better withstand the heat of the radiator.
3 perimeter, 25 % gyroid infill, no supports
Assembling:
Push the top of the hook/protector into the desired rail and then push the lower side into the lower rail.
Move the hook/protector along the rail to the desired position.
Measurements:

The author marked this model as their own original creation.