Nothing really special about my weights - I used several bags of lead shot. The tricks I incorporated, included a movable fulcrum on a sliding track to maximize the downward angle of the weight, a sling for the tennis ball and the spring/flex of a steel rod also adds to the power equation.
The largest one my brother has built was of a much simpler design, but it was truly a weapon of mass destruction, compared to anything we could legally buy and shoot. He had a couple of old junk cars from a mechanic neighbor's salvage "garbage" that he moved over to the target area. It was amazing to see what a concrete block or a hardened bag of cement would do to a car.
It's all just boyfun.