Plain masking tape is also something you can work with fiberglass just like cardboard.
So get some cardboard and form it to shape and use masking tape to help hold the shape you are interested in. For compound curves use non corrugated cardboard and cut slits in it and tape them smooth. Construction paper also works good for some shapes. The resin will soak through the paper and the masking tape just like the cardboard. So after you get the shape like you want it put a single layer of fiberglass mat over it and resin and let it harden up enough to hold it's shape then put another layer on to make it strong enough to work with.
After that take it off the car and scrape any excess cardboard and tape off the back then put a layer of fiberglass over the back side to seal the cardboard in. If there are some areas that are not right you can sand them down where they need to be and repair any holes you cut in from sanding bad places out. Usually it only comes out a very tiny bit thicker but is just as strong making the parts this way. If you want it to be real thin just use thick paper like poster board paper instead of cardboard.
Also if I remember right, powdered graphite is what you mix in the resin to make the fiberglass black. That way it is black all the way through and scratches won't show up easily in it. It also hides the cardboard so you don't see it through the resin
