So I have used three different materials for my past Spartan visors but the only one I would reccomend is what I did for my latest one. My first one was sunglass lenses, it worked but limited vision, it was my first cosplay so I had no idea what else to do lol. The second one was a motorcycle visor, but I had to use a ton of hot glue to hold it in place and that only worked because I built the helmet over a bike helmet, so it was a bit clunky. And lastly, I used some Cricuit brand flexible plastic that I put car tint on. All of my methods have been flat, so if you want something vacuum formed, that is not my expertise but the take away is that any flexible plastic will work as long as you tint it before putting. For the love of all that is holy, if you are using a plastic tint roll, put the tint on first, trust me!