1) How hard is it?
2) Who makes the kits and where can I buy them?
3) How exactly do they work and how do I install them in the ball?
4) How big a chance do I have to screw something up?

If it matters, I drill a shallower thumb hole than most. My thumb hole probably doesn't extend into the ball 3 inches. This is sort of by design, as it keeps me from ramming my hand in the ball as far as it will go (and trigger a sticking-thumb problem in the process), but it's also due to the quill stroke on my drill press being a little shorter than it should otherwise be.

Jess