I've heard that painters plastic works as a release media.
A lot of websites have info on plug molding for fiberglass shells. Not what we want though.
This guy
Using Fiberglass/Foam Sandwich Composite builds some interesting projects. Don't cry, the aero potential of his builds gets ruined by style scoops and flares. Like the attached flow killing lump on the XR3.
Anyhow........ be cafeful of your foam-resin matching. No bondo layer = much less weight, and cost of course.