My scultpture textbook in art school had this basic design, as a woodcarving clamp. The bowling ball had a double ended lag screw on it; you just screwed on your hunk of wood and started carving. Instead of disc brakes, the model there had a stand made of 4 4x4 posts hinged at the corners of a 2x2 square of plywood with opposing members joined by threaded rods running through pivots halfway up. The bowling ball rested in notches cut in the ends of the 4x4 posts. Basically a big gripper-clamp for the bowling ball. Not nearly as quick to adjust, but then wood carving isn't a fast process.