Here is a thread on Zero helmets.
Also, I am definitly positive that there is info on his helmet in the Geass thre
ad, you just need to find it.
Most people use plastic to make the viser and tint it later on. Plexiglass is the common material used for Power ranger helmet visors, so you may want to look into that. Other methods in costructing a Zero helmet is paper mache, cardboard (featherweight's tutorial is pretty useful), modifying an exsisting helmet, and fiberglassing.
"Now whose the badass biotic, bitch"
Female Ghost (Starcraft II)
Four Eyes (Resident Evil: U.S.S Delta Squad)
Samantha Byrne (Gears of War)
Zatanna (DC Comics)
Kasumi Goto (ME 2)