I went and picked up 8 of the larger ones. They are just about the right size if not exact.
The smaller ones were the size of a baseball and the larger about softball size if not just a touch larger (Not by much at all, but I do not have a softball here for comparison)
The larger ones were $6.50 each and the smaller ones were $5.50 each.
Here is what I have done.
When I bought them, they have steel rods screwed into them.
This is what they look like..
I removed the rod to see the exposed screw that is attached to the ball..
I noticed that the threads are soldered into the ball.
I placed the threads in a vise and twisted hard, thus breaking the solder.
It seems that the threads have a flat top on them that are inside.
I let the threaded part slide inside of the sphere and used a 3/8" drill bit and drilled a hole..
And here was what was attached..
Now you have 2 ways to get around this.
1. Grind down the threads to get it flush so you do not have to drill the large hole (I wanted the hole as I want to do the version with the drill bit coming out as well as the blades so this worked perfectly)
2. Use a 3/8" drill bit and drill a hole. The "screw" comes out with a hole that large perfectly.
Hope this helps you all that will be doing this.
Brandon