Any plastic bowling ball will do. Pick one that you like. Ebonite's Maxim series is good, made for spare shooting like the Track's Flaming T, Roto Grip's Spare Tire, Storm's storm balls, a skull ball, donald duck....anything. Just drill it neutral, and make sure you don't have any pitch to your grips. It's what you want and can afford. And they do come in handy. Lots of plastic bowling balls cost in the $50 dollar range and they are easy to drill as well. Most people maintain the same weight as their regular bowling ball, though dropping a pound works as well. Your choice.
Chuck