Fantastic? Thanks, but I'm hardly a professional.
The top of this helmet, the dark blue part, is a hard baseball cap that I cut. The rest of the helmet is made of sheets of styrene or some other plastic with a craft foam backing glued using hot glue. The forehead spike is made from a plastic plate that I cut and rolled into a cone.
I would love to see pictures of your Alphonse cosplay. I'm still a far ways from completing mine.