Funny, I'm actually doing a double-layer visor for my Reach Hazop helmet so I got sucked into checking this out. We'd tried doing this with a Dremel (but using a different bit) and it was looking really bad.
So, I'm going to pay a few bucks to have someone with a laser cutter do it for me in 30 seconds.
Got to love what technology will do for us. It's so easy that it feels like cheating.