I am aware of the fixture out there, and I don't think it wound be hard to replicate. Trace a bolt on a piece of paper to start, then draw the axis you want for turning the knob. Just mill v-grooves and clamps to hold the bolt body, and set the thing on center.
I never bothered with the fixture. I like to be able to choose a straight bolt or an angled bolt. I just grind the bolt knob into a square, put it in a vise, file it to 5/16 and uniformly round (its easier than it sounds). Then run a thread die over it. You are probably gonna locktite that thing on anyway, if not J-B or braze, so the threads just need to be reasonably formed. It's a 30-minute job. Making a fixture might take a few hours in comparison. I'm not running a factory, so I'm still content to do it the old way.