The counterweight box is made with a simple frame of 1 1/2" x 1/2" members joined together with 1/4" dowels, then covered over with 1/8" thick strips placed vertically. You can get the idea of how it's put together in the bottom picture.
Pretty much everything on this is done by hand, the exceptions being ripping the frame members on the tablesaw and drilling the dowel holes. NB: when drilling the holes in the throwing arm for the axle and the counterweight box, do so BEFORE tapering the arm. That way it is easier to make sure you drill straight. Use a drill press if you have one. Of course, it's not really necessary to taper the throwing arm, but I think it looks much nicer that way.
Click the thumbnails to see the full images.
Here is a sequence of it firing.It can throw a golf ball about 100 feet. I only got one try throwing the golf ball, though, because I only had one, and 100 feet down from the trebuchet there is a bit of a swamp right now, as the snow melts and the ground is still frozen. Maybe I'le find it in the summer, after it's dried up down there.
This is the firing apparatus. I have pulled the strings aside to give an unobstructed view. I made this out of a couple of 10d nails.
Here is a joint between a crossmember and an A-frame member. It is a half lap joint with a dowel to reinforce it. I think it's probably a bit of overkill, but it looks cool.
And here is what's in the counterweight box. It's about 11 1/2 pounds of stones.