Step 1: Parts needed
Eight cheep white rubber erasers--$1
One roll duct tape--$1
Plastic utensils --you probably have them already, one knife, one spoon
The erasers are just for absorbing the impact of the mouse trap bar so that the catapult will throw projectiles in the right direction. There are probably many options for substitutions.
Any normal mousetrap should be adequate, the same goes for the plasticware.
Use only the finest duct tape.
Step 2: Safety First
Step 3: The Rubber Wall
Step 4: There is a Spoon
Put the flat part of the plastic knife across the snapper--this will act as a solid anchor to tape the spoon to and increase the durability of the catapult.
Place the handle part of the knife behind the spoon to support it as it flings projectiles at your enemies. Secure all parts with duct tape.