Just wanted to add a note about using balls made of substrates like Acrylic;
The juggling site adds this bit of info:
"Don't forget to protect your contact juggling ball from dents and scratches by also getting a felt-lined canvas Contact Juggling Ball Bag and Acrylic Polish!"
Any material that "dents" or scratches easily is not going to endure for long as a trackball. The specific gravity of acrylic is also a fair amount lower than that of the materials used for trackball balls, which makes them lighter. This will make a difference in performance.
If anyone is interested in some really nice 3" replacements in translucent, bright Red, Green and Blue, in the exact weight and durable material of the original white balls, feel free to drop me an email. I can tailor a kit using these, with lighting as well, just like the EI upgrades.
RandyT