This is easy to build and requires very little work.
I've been able to launch rocks as high as 30 feet!
(I was going to show the final result but it was very hard to see the rock.)
Step 1: Parts
1 Piece of Wood
1 Bottle cap
Cut two slits the size of the cable-tie.
Feed the cable-tie through the top of the cap then through the second hole.
Place the wrench on the board and make a mark where the front of the wrench touches the board. (Make sure you leave room for the adjustment screw.)
Then chisel a notch to hold the wrench stable.
Screw the the wrench in place with the U-bracket.
Finish by tightening the cable-tie around the wrench handle.
The more you tighten the adjustment screw the stronger the catapult is.
Now take it outside and have some fun!