mattman said:A "hole-saw" and yes, they make them in all sizes....they just attatch to the end of a drill.
The only problem I see with this is I doubt the balls would spin well, if at all. Now, if you made it to where the bottom was stationary and the plywood spun, that would work. If you just like building and want it to look professional, this might be the ticket. Although, I think it's way over designed. There is nothing wrong with the balls touching each other and as already stated, they don't touch each other all that much anyway.
ok, then ill probably will go the easy way.

saves me work.
well it doesnt cost much to try it either. if its overdesign, just take it off.