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.