I am building an epoxy fibreglass body for my Geo Metro with pink styrofoam for the core material.
Instead of a custom made bubble, I am using a standard small airplane bubble.
There is a lot of info out there on making composite bodies.
Here is a good project.
AutoSpeed - Building an Ultra Light-Weight Car, Part 1
AutoSpeed - Building an Ultra Light-Weight Car, Part 2
Volkswagen L1 is another great design.