Hey Melvang, good to see an old face around here! Things are pretty quiet here, but let's see if we can breathe some life in.
How big is your yard? The usual Tennis Ball Trebuchet design to default to is (the late) , which I've been hosting on teamurbansiege.com for a number of years now. But that will throw upwards of 100', maybe too big for the yard? If that's too far, then something in the size range of what you mention ought to do the trick.
If you go for a FCW design, put wheels on the frame, it works better, and looks cool. The kids will probably love that it rolls back and forth