The final version of the NERDS ball magnet uses wedgetop conveyor tread, as seen here:
The frame member the ball sits against is set back 3" and covered in felt, the idea being that the roller sticks to the ball and the ball slips on the carpet and frame member. Ideally the ball is touching all three surfaces at once.
Something to keep in mind is that these devices are very finicky (at least from what I've seen/heard). You need to play with the dimensions a lot to get optimal performance out of the mechanism, and even then it can be easy to loose the ball while turning.
There are some more pictures of 1726's device in this gallery:
http://photos.project1726.org/