I cut a piece of masonite out in the shape of the fin. I didn't get it exact as you will see in the later photos. But it isn't anything that I can't cover with Bondo.
I epoxied the fin onto the helmet. It only touched in the front on on the back side top of the helm.
As you can see there is a gap in-between the fin and the helmet.
The back doesn't even touch at all. But this can be fixed.
i forgot to take pictures of the bondo process. I smeared bondo into the crack and once it hardened I sanded it down smooth. Then I added a coat of paint to the new fin and all of the bondo areas. I cracked the bottom of the helmet in two spots. One was on the chin, and the other was on the right side. I saw the one on the chin and fixed it. I only found the one on the right side when I painted it. But that is where the decorative stuff goes so I am not too worried about it.
I am really excited about the way this is turning out. For my first attempt at anything like this I am really pleased. I am not quite sure yet how I am going to get the decorative stuff on the sides or the air vents on the top. The pepakura file I had didn't have those pieces. I guess I should have looked at that first.