[MrForbes]

You probably want to make the system so that the ball is being driven continuously, and a belt of some type is a pretty good way to do that. To do it with a series of rollers gets kind of complicated, but it can be done. One way is to have the rollers on both sides, and make the path curve back and forth, so the ball is always in contact with a roller. This takes more rollers than the two that are required for a cord type system.

Fewer moving parts usually means you have a better chance of getting it built and working in the short build time remaining.