I cut visors for my commissions from polypropylene welding shields. These are a dark smoke gray color, and they're lighter and more flexible than formed acrylic. Here's a shield as I receive it.
I have some visor templates that I've made to fit the various types of helmets. To cut the visor, I lay the template onto the welding shield.
I mark around the template with a silver Sharpie.
A few minutes with some tin snips gets me a nice custom fitted visor.
I'll mount the visor later today, and this helmet will be finished!