How to darken a drop down visor?



I have a modular helmet with a clear face shield and the drop down tinted visor. The visor isn't dark enough for my eyes, and I find myself squinting and getting a headache in the glare. Saturday Bike and I went out for a long ride, and I wore sunglasses which helped with the glare, but was really uncomfortable. The helmet made them push too hard against my head so I still got a headache anyways.
I was thinking maybe I could get a piece of that window tint stuff they sell for cars, and...

How to darken a drop down visor?