OK I've decided to use billiard balls for my Newton's cradle.
But I would rather not drill into the balls or try to glue things on
them.
How can I hold the balls in position?
This very large version has some rings, which is what I am
considering:
http://i48.tinypic.com/1euurq.jpg

But people are likely to pull the balls out if they can, and I would
rather have a couple of large rings, one on each side, centered on the
point of impact. I'll have to tie them together somehow. Cable ties?
I'll probably use heavy monofilament nylon to hold the balls in line.
Unless someone thinks of something better.
I have to make this in the next few days!