You can get remnants of plexiglass in red from a sign shop (that deals with colored plexi for cheap), leave the film on, plexiglass can be bent by putting the piece of plexi in boiling water for under a min or use a heat gun (I like the water method as it heats the lens equally), use silicone tipped tongs (as to not damaged the softer lens) other wise you'll burn youself, don't use pliers as they'll mark up the lens, once pliable you can press it into the inside of the helmet where it will adopt the curve, keep pressed until cool, use light gloves for this, cotton garden gloves (they'll help protect your hands while the lens cools) will do the trick. Once you're happy with the shape, make your final trims (sanding block is handy for final touches), before you're ready to stick it place remove the outide film, remove the inner film at the end (always leave the lens protection film until the end).