Here is the Ebay ad. He doesn't say how thick, but I would go 1/4". I think if you go 3/8" thickness, you might have problems installing it in your frame.
Scratch and Dent Lexan Windshield for AC Cobra Roadster Replica E | eBay
I have Lexan windwings and have literally beat them with a hammer. They are regular Lexan, not M10. They are not scratch resistant, but they are pretty hard to scratch. You can get Lexan in various shades also. I am not sure if you can with M10, but if so, you might want to put a little tint in the color.