Here is another STEMFIE catapult project with fewer parts; therefore, it will print in just a few hours. You can press and hold the catapult's base on a tabletop and shoot the ping-pong ball by quickly pushing down the beam. 

You can download the Blender and build plate file from this link


STEMFIE.org Simplified Desktop Catapult v01

STEMFIE.org Simplified Desktop Catapult BUILD PLATE