I got a green welder's shield with my fiberglass helmet. I used long scissors to cut it, and found soon after that the scissors had scratched up the visor. Not good. Use short scissors...
I happened to have "The best T-Visor" from eBay, and it works well. I had to trim it a bit with a Dremmel-like tool to fit it around the cheeks, and I shortened the top edges a bit. I didn't have to reshape it's curve, though.
I drilled right through the helmet in four places: on either side about a half-inch out from the eyes, and about a half-inch inwards from the corners of the "T". I put in four screws, and Bondo'ed them in place. I installed the visor with some large washers and screws. The visor is flexible enough that the screws pull it nicely into shape.
I then Bondo'ed around the edges to make it flush with the visor. The Bondo ruined the visor, so I have a new one on its way.
The visor is now held very securely and flush without any glue. All it uses is four screws, and the visor is easily removeable.
