Here is how you can use a heat gun ( preferred) or a hair dryer ( works but slower to heat) to heat and soften the urethane so that you can CAREFULLY cut into your mask with an X-acto knife. Please please, cut slowly and away from yourself!
Be very aware that there are thinner and thicker spots in the mask you will need to cut into. Going from a thick section to a thin section may cause your knife to SUDDENLY accelerate and cut a finer which is exactly how i just ended up with five stitches in my left thumb. It sucks. Wear a cut resistant glove!
