...How did you hold the part? Did you find the grinder getting away from you at all, or did it cut away material without any issues?...
Well, woodcarving bits are by design made to be less prone to grabbing (i.e. kickback), but that really should apply to most die grinding tools. It's the angle grinder chainsaw bits that are so dangerous.
In my case, I actually ground out a slot into the interior or a rubber mini-golf ball, and grabbing a sphere (or hemisphere flat-side up in my case) is notoriously difficult, so I did the stupid and held it in one hand and held the die grinder in the other. Still have all 10 fingers.
If I were doing what you're doing, I'd either hold the pad (while still mounted on the Y) in a bench vise, or if it's too big to do that, clamp the stand to my bench, and then two-hand the die grinder with at least one palm well supported on something, taking gentle passes. The lighter a touch you make, the less likely it is to grab.
I modified my jack stand pads (Hockey Pucks) with a hacksaw blade
That's feasable. I used to have a rasp made out of hacksaw blades (
Shinto Rasp) that I would expect would work well on rubber.
A hacksaw is just a very narrow rasp. The hardest part would be keeping the blade from falling into previously cut grooves. Power tools mostly avoid that trap.
