Hawkchucker, there are a few plans out there that are both free and pretty good for your purpose. Ripcord's Tennis Ball Treb is elegant, but has some funky compound angles in its frame and calls for odd sizes of wood (backstory, he had some hardwood scraps that he ripped to the sizes called for, and the angles are required to get the sides tilting inward so the axle would have a short unsupported span). It works really well, though.
http://members.localnet.com/~lorilee35/Another is the Ingenium Medium Trebuchet (there is a small one, too, but no big one). Its frame is simpler than the Ripcord model, but Rip's arm works better and the counterweight is simpler.
http://www.tasigh.org/ingenium/medium.htmlFor the best of both worlds you could combine the Ripcord arm and CW with the Ingenium frame, though you might have to fudge dimensions a little to make them fit.
I have my own designs, but no formal plans.