Warning: requires lathe work to make a metal washer which is 2mm thick overall, has a 9.5mm central hole and a stepped external diameter. One step diameter is 19mm x 1mm thick and the other is 21mm x 1mm thick.
The bolt is a standard 3/8"-16 UNC that is used on larger cameras and telescopes. The ordinary camera bolt (Canon, Nikon, etc is the smaller 1/4"-20 UNC).
Camera knobs (even a cheap-o Desmond D-ring) goes for $6+. Other brand name levers, locks and knobs are very expensive. This one is for a telescope and cost at most $0.50 in materials.
I wanted to try embedding a bolt into a print, but compromised by capturing a metal washer. The knob is printed "washer-side" facing the print plate. The steel or aluminium washer is inserted just after the recess is printed, such that the metal is flush with the bottom of the knob.
Ensure that you have applied hair-spray to the washer surface before inserting, or the plastic extrusion laid on top will not stick to the washer! Also make sure you have pre-heated the washer by placing it sit on the heated bed before inserting.
The washer can't fall out, as it has a stepped edge (not 100% essential, but a nice-to-have feature). In practice, the plastic knob only exerts force onto the bolt sides. Clamping forces once the knob is tightened are 100% metal-to-metal (which is why a metal washer was used).
An objective was to make sure that zero 3D printed plastic bits were crushed when tightening up equipment using this knob.
I've not pushed the tight-fit plastic cap home for now, as I still need to machine the bolt shorter and make a slot for a retaining washer. I'll probably epoxy on the cap, then sand it flush and paint. Form Futura ABS pro sands down exceptionally smooth.