Step 5: The engineering part
It's important to get the flap working right. It needs to move easy, yet move back across the hole. Vacuum flow will make it shut. But the impact of a ball hitting the flap will be enough to make it open without slowing the ball by a lot. Oh yeah, the flap will wear out fast if you use cardboard without some kind of shield where it will get hit by all the balls. I wore out a piece of cardboard on prototype one after only 150 shots. The tape is also the hinge for the flap.