Guys! I saw this on a cartoon. It's easy, you just have a friend hit you on the head with a frying pan. It will bend into a perfect form around your head!
Ok, don't try that.
I would say find an already made helmet to take the dome from, or find something to modify. The whole idea of doing it in metal is because its metal! If you did part of it in fiberglass, it's just not the same.(not ripping your idea apart, A'den Kyramud) If you go metal, then take it all the way, IMO.