Instructables user rainingfiction decided to spend a recent weekend making a Skyrim glass helmet. They documented their process step by step. Most of the materials used were already on hand, so in addition to being a relatively quick build (it took 24 hours), it was inexpensive. Rainingfiction used a couple different kinds of foam, contact cement, super glue, paint, and a few other items.
The build started with a head cast. Rainingfiction used tin foil to their head to get the general shape, then they put duct tape over the foil and drew the pattern. For the fin:
To make the fin, I contact cemented 3 pieces of 5mm foam together. I drew out the shape of the fin in the front of the helmet, and dremeled an angle into it (pictures show where I do this). For a nice detail piece I two triangular shapes out of 3mm foam. I lightly scored a design into the piece, heated it with my heat gun to make the lines more prominent, then super glued it to the fin.
I cut 3 slashes all the way through the fin, and heated it with my heat gun as well.
The second fin is made the same way, sandwich, dremel, design.
You can see a couple of in progress pictures below.
Get all the instructions at Instructables.