Sorry I just saw that nobody has answered the question.
I copied one off of the the RCBS Rock Chucker that was on the bench next to it.
I'm in the machine shop for work several days a week so it was easy to ask to make one.
Seems to remember a 1/2 rod threaded on one end and a 2 inch plastic ball from McMaster-Carr.
Warmed it up and put a couple of bends in it and it took care of my needs.
The stock wooden handle does the same with the round end. You can palm the handle as you go
thru the motion unlike the hold on the roller handle.
Sorry for the slow answer.