Ok, so here’s how it starts. The helmet looks basically like a ball, on top of a sleeve/collar.
I’ll start with a kid’s rubber-ish ball. I started by trying to stick the ball on a piece of cardboard tubing used for making concrete forms (it’s available at home improvement stores). But the cardboard cylinder does not “flare” out correctly. I tried to cut a ring of cardboard, then slit it and fold it in on itself. But it’s just not right.
The shape from the “globe” of the helmet, down to the base where it connects to the collar, is not a cylinder, it’s a conical section. I think a different approach is in order. If I pick point on the helmet where I think the sphere starts to flatten-out to the conic, and get a diameter there, then measure the base of the helmet, and those two diameters will form the top and bottom of my conic section.