This model is a rework of the design found here: https://www.printables.com/model/168914-12mm-rod-low-profile-clamp
The original model on printables had under-defined sketches, and needed a few tweaks. My version is designed for a 4mm cap head screw to secure the rod, and 5mm button head screws to fasten the clamp to 20mm extrusions.
Also on Grabcad: https://grabcad.com/library/rod-clamp-12mm-1
The author remixed this model.