I didn't get to build the trebuchet and my partner who built it with the help of his dad built some lazy traditional (c-word). Shot tennis balls, hand balls, and a golf ball about 4.5m, 6m and 7m with a 2.5kg counterweight. We shot the projectile at States 5m while other teams were getting around 20. I only learned what a FAT was after the competition. Next year I better use that, especially in New York. It was embarrasing because I impounded and talked to the proctor about it.
Good luck, and I'm sorry I don't have enough experience to help you. BTW, what distances are you expecting to get from it? I can help you make the graph if you think you'll need it to impress the judges or you just wanna look smart and prepared.
Goodbye Science Olympiad, until next year.