sorry to rez this older post but had an idea.
Hollow is out through the bottom of the finger holes as much as you can to remove as much weight as you can. Fashion some scrapers etc at an angle to keep from having to enlarge the diameter of the holes much like wood turners use when making narrow necked vessels.
Now you have a fun bowling ball weighing next to nothing that you can pull pranks on people at the center with in a much safer manner than you could with a 15 pounder. Toss it at an unsuspecting persons lap, offer to let someone throw it and watch what happens when they try to pick it up with the force they'd use on a normal weight bowling ball, etc etc.