I also like to use tennis balls for practice casting!
I use the tennis balls the way they describe, with a slit in one end and a tiny hole in the other, where I just stick the shock leader through the hole, then loop it right through the sinker and pull it back into the tennis ball!
I have them set up two ways, one left normal, this will teach you how to cast slowly and smoothly the hard way! If you try to overpower it, you will almost always overrun and that helps with your form!
The other tennis ball is set up the same way, except it’s completely covered with bright electrical or duct tape to help it cut through the wind!
Also, if I woods a few or crack-off, I can find them!
EA