mine looked very much like that, except I did a slightly smaller area, maybe 80% the size of his.Werner Berry used both tape and a Dremel for the magnets on the original BerryBot3D. Here's a photo.
http://forums.reprap.org/file.php?1,fil ... .27.35.jpg
I think mag > ball < mag would not leave enough room for the arms to move around ... or if you left enough room, then the contact point would be so small that it couldn't hold the ball there.Why not just use two magnets IOW "magnet-ball-magnet" think the magnet are easier to glue than the ball