Our team need about 100 knobs to complete a project. Trying to buy the knobs locally was costly and limited stock. We decided to draw it up and print them. This was my first attempt at creating threads. The knob is 50mm with 5 arcs for finger grips. The knob accepts 1/4"-20 x.472 insert nuts.

Printed on the Replicator using MakerWare. ABS, 100% infill, 2 shells, .27 layer height, 230 extruder, 110 platform.
Once complete simply screw the insert nut into the knob. There is an upper end stop that you can really twist the nut into, to make it stay. If needed you can use an epoxy to make sure it does not back out.