Well, I'm still looking around for a pastic ball that has a 4.5-inch diameter. The 4-inch one MIGHT work, but it's a really tight squeeze and won't leave much room for movement as it is. I've checked McMaster-Carr (
www.mcmaster.com) since they have a ton of random supply stuff like that, and they only seem to have 4-inch and 5-inch, and I've already checked the 5-inch, which is too big for the housing. I might check it again to see if I can make it work somehow.
If anyone knows where I can get a hollow 4.5-inch ball, that would save me a lot of time... if I can figure that part out, then I'm away to the races