I actually did this quite a while ago, before I built the ball turner, but I kept forgetting to take pics.
At this point I hadn't turned a longer taper yet. My compound will only to about 2 inches. In any case, I figured it would be a good idea to rough in the taper before actually cutting it. I just eyeballed it.
It turned out pretty well. Sorry about the white balance.
I just used the tip of my indexable tool to cut the grooves in the top and sides.
I also decided to play around with the taper and stepped the sides.
You can see the hole is quite big. At this point I hadn't tapped anything either so I wanted to play it safe. I turned, bored and tapped a length of delrin rod which I expoxied into the large hole on the knob.
I plan to do another one with the ball turner, so stay tuned.