Created from scratch based on this design on Thingiverse with some tweaking. Designed to be printed without support: please turn the print like in the slicer screenshot (cable rail point up.) Can also be printed in orientation as came in the file with support under the cable rail part (cable rail point forward.)

Uses 2x M4-10mm screws and T-nuts to mount to the side of the extrusion.

Provided STEP file is exported from Rhinoceros. I don't guarantee compatibility with other 3D programs as I don't have or use them.