Found these on McMaster Carr. I plan to use them under the car to attach fiberglass using 1/4-20 rivnuts.
http://www.mcmaster.com/#90015a410/=10kzmsw
The head is about the thickness of a washer (.110") and the head diameter is .755". Seems like the way to go for fiberglass.
Maybe add a little purple loctite for good measure.