just that the vacum forming is kinda expensive because you have to buy wood, a saw if you don't already have one, sheets of plastic, and a vacum if you don't already have one
or you can ask someone on the site to vacum form a visor for you, just you either have to send them your visor or tell them the scale of the helmet so they can print out and make the visor
another way could be to heat up the sheet of plastic and when it gets hot just stretch it over the helmet and have your glue gun ready and hot glue the inside
hope this helped and good luck =D