Doesn't look like it could expand past the border of the Magic Rack.
Look at this, should start at 58:00 and what happens after the break.
At certain point it looks like the one the referee uses, although it also clear like the one on eBay, can rest on one side of the rack but can reach to the inside of it.
https://youtu.be/tZOMD8tl7Iw?t=58m
Maybe I'm wrong.
the one in the video is different than the one that is on ebay, however, both can do the job nicely, imo.
Whatever you use, it can be a delicate job moving, then replacing a ball, or balls, while sliding a magic rack out.
I have the same one that's on ebay and it works great. It's precisely cut to perfectly fit onto a 2 1/4" ball and it being clear allows you to see through it as you are setting it in place.
It's small, lightweight, and costs $3 incl shipping. To me it was a no-brainer.
The one seen in the video appears to stay further from the ball it's marking by touching at center ball.
The one on ebay slides right up to the bottom of the ball on the cloth but in my experience the rack can still be easily slid away.
Maybe get the ebay one and if for any reason you don't like it, then re-purpose it as a doorstop or doggie chew toy or similar. :grin-square:
All kidding aside, I think you'd like it.
best,
brian kc