I have a helmet from 1966 that came with a Yamaha. It is like a football helmet inside. It's a fiberglass shell with canvas straps and a couple pieces of foam inside. Very light and very cool since it lets air flow through around your head. It was a stock approved helmet and a 1/2 helmet.

Some pretty light and thin helmets on Ebay but don't see anything as thin as mine.

I learned all I need to know about life by killing smart people and eating their brains.
Eat right ,Exercise ,Stay fit, Die Anyway!