You can get the acrylic grey 1/8" thick in the same tint as silverstalker linked above.
This is what I did with success:
-Preheat your oven to about 250F (ish), put the acrylic on a cookie sheet and bake for 6 or so minutes.
-Bend the glass over the face of the helmet( put a cloth over the helmet so the heat doesn't damage it.)
-Once it's bent to your liking run it under cold water to set the shape.
Also...Make a visor template with a piece of cereal box cardboard so you can get the fit right before you go cutting on your acrylic.
....imo cut the acrylic after bending it.
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