I'm not a historian of ancient Greek armature, but if I were to fasten a bunch of horse hair to stand up like a Mohawk on top of a helmet chances are I would drill a bunch of holes of roughly .5 cm diameter systematically throughout hardwood base and stuff each hole as full of hairs as I possibly could. It would probably come out looking something like a shoe brush.
Of course, I have no idea how they were made; again, I'm not a historian. I just don't see what is so obviously machine made here. On the contrary, I would be surprised if this helmet wasn't in fact custom made by hand by the costume department. It would be very unlike Nolan to buy a mass produced Halloween costume from China.