what i did was take three sheets of transparent coloured plastic and layered them yellow, red, then blue. this will blur your vision very much but leaves the reflective shine so that others will not see you. laminate it (will cause bubbles to form) or find a way to put all the sheets together as one and transfer the pattern of the visor onto the sheets and cut, paste, and insert.
So far I made a visor for a friend using a face sheild and gold tinted self adhesive industrial window film. it came out looking very nice!
I was thinking of using those metal mixing bowls you can get at walmart, as they have a nice compound curved bottom. and in essence using them as molds, squeezing a pice of lexan in between them in the kitchen oven. with the window film applied to the plastic. prior to cooking it.
Thats just one idea.
The best one would be vacuum metalizing if it were possible to make it optically clear, and not cost a rediculous amount.
2 3liter soda bottles
1 roll of gold reflective tint.
1 soldering iron
cut the soda bottles into the size needed to make a visor for your helm. cut the tint to the same shape. next sandwich the tint inbettween the soda visors you cut. take the soldering iron and melt the sides sealling your visor.
I had the same question a few days ago and started brainstorming and came up with an idea.
I thought that you could make a positive mold outa clay or something making the designs in the visor and stuff.
Then you would vacuum form it with clear plastic and apply gold tint to that.
I think it would be cool to make my own visor but it would be so much more expensive than just buying a visor on ebay or some where else.