I purchased a used roof rack, and 3 of the 4 necessary nut knobs were missing. I've simply used nuts, but there's a danger of scratching the car while working with a spanner! I designed replacements based on the remaining one, and printed as necessary.
There's a captive nut inside the part. I didn't add any pause or anything fancy like that, but I made sure to print more than one piece, so I could add the nut in one while another is being printed. Simply keep an eye on the layer number, and insert the nut while the printer is working. To do this successfully, I made sure to leave some space between the parts, so that there would be space to get access while the printer was working.
I work in FreeCAD, and I've added the design for anyone who's interested in modifying or learning.
I've tried adding a GIF animation of the layers, but it's not showing on thingiverse yet.
Remember to add the nut at approx 8mm layer height. Inspect your STL before printing to find the last layer before the nut hole is covered. if you are printing at 0.2mm layer height like me, that's layer 40.