I've been building a custom helmet for myself since April 2012 during spare time.
The cardboard helmet has exact size of my head. ^^ I can put it on and off.
The next step I'll do is to construct decent mold over it. Then I'll cast a shell, put liner, and install scales as outer shell that deflect and absorb impact like below image.
Conventional bike helmet is bulky, needs to be replaced it when it got heavy impact for safety reason. It works as one-time protection shell. To me this is really waste of materials and money of riders.
So I figured I would build a helmet that has a layer of scales that can be selectively replaced after impacts like the body armors in ancient time.
This part consumed most of time.