I haven't cut apart your particular helmet but many have a plastic skeleton the foam goes around. Even without the skeleton, helmets are designed like those balsa wood trusses you built in junior high science classes. The strongest lightest thing you can manage. If you take out a piece, especially a piece large enough to add meaningful ventilation, you will completely compromise the strength of your helmet. Don't do it, you'll ruin your helmet. Get a better vented helmet and save your old one for the fall weather.