This is yet another customizable knob / wingnut. By default, it has a captive nut inside. Just pause your print when the nut cavity is almost finished (check in your Slicer preview the depth at which it finishes) and insert the nut. It should fit well as the filament will be soft. And then continue the print. You will get a bit of messed up filament probably, but threading the bolt through the knob will clean it up.
One thing I tried in a recent print is to put some glue on the top surface of the nut, using the same purple glue stick that I use for bed adhesion, to make the filament stick more neatly to the metal. It seemed to make a positive difference.
The STL files are for a 1/4" bolt and the standard size nut going with that. With the default sizing, pause the print in the last layer before z = 11.5 mm.
Feb. 22, 2019 update: Minor tweak to neck.
Feb. 16, 2019 update: Added chamfering, thickened horizontal walls.