I have been using 2 sheets of polytarp re-sourced from a dogfood bag as a backstop in my catchbox. So far nothing I have shot has penetrated it. (5/8" marble, M10 hexnuts, 3/8" steel, .50 cal lead). Tonight, I strapped an empty Aldi rice cake bag to my catchbox. Not marbles, nor hexnuts, nor .50 cal lead could penetrate both layers of this humble snack bag. From point blank range, a hexnut embedded itself into the catchbox but just bunched up the mylar and pulled it in with it. .50 cal lead could tear the first layer, but was stopped by the second layer, falling into the sled I use as a catch tray below the backstop. Short of anchoring all 4 edges in a braced frame, how do I ventilate this stuff?