Handle finally finished. Main problem was trying to decide on the length of the bars. In the end, I decided on the maximum they could be, without interfering with the Y feed. It works really well to move the jaws from fully open to closed. Only downside is the weight, (but that might assist with the inertia to spin.) I was going to drill a complete set of 12mm holes, but could not calculate the angles and depth when they would start to break into each other , so interspersed the 12mm's with 6mm holes.
As I have yet to work out how to put words between photos, here's the list:
1. position of handle when jaws fully closed
2. back view
4. clearance shown dictated max length of bars.
As yet, there is no sprung-ball retainer, (as advised) but will try without, and hope the experience is not too painful.