Note: Not ANSI or Snell approved for protection against errant meteorites, asteroids, or light sabers. Wear at own risk at being called a geek. Light sabers not your thing, maybe make this instead.
Step 1: It's just a shell game...
It's a hardhat hat over a hardhat.
Pros would probably fiberglass or vacuum form this thing. No need to if you are
You will need a regular standard hardhat to build upon. Get the one with the ratchet headstrap adjuster which makes it fit real nice for a few bucks more.
You will need some cardboard and paper.
You will need a beach ball or something similar to use as a half dome form. A giant salad bowl would work too.
Black gloss paint.
Utility knife, scissors or strong shears to cut the cardboard.
Caution: Be careful with sharp things and black paint gets all over the place no matter how careful you are.