Had something similar with my grills, I bent them the opposite direction slightly and left them there for awhile (few weeks). I didn't think of adding heat, but it would probably help speed up the process; maybe with a heat gun or blow dryer, if you don't have a heat gun. If you don't have a blow-dryer around, well then I'm out of ideas. Maybe a toaster oven on the lowest possible setting (<~200 F). Another option is dip the bottom in boiling water for minute or two and try to shape it after you pull it out, this is how we used to shape mouth guards. Whatever you do, take the lenses and foam off FIRST!
Just to cover my butt, I'm not liable if you ruin your mask
These are just ideas. let someone else chime in case I'm crazy