Like Greg said...use DOT glass.
In reality you would not want to use anything but glass. While it sounds good to have something that won't break, because it is plastic, it will scratch. Look at the plastic headlight lenses on cars...they look fine the first year, but after awhile they get that smoky look to them. Do you think you really want to be looking at the road through that?
I forgot about scratching plastics. I worked at a local carwash for two-plus years, and I did many headlight restorations, and they are not fun at all.
Are there some types/brands of glass that are better than the others?