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Shape it with foam and then fiberglass over it. Don't use the cheap chopped strand mat. Use the fiberglass cloth. Otherwise you will have a lot of sanding and filling to do. You can get cloths in weights from 18 oz to 2 oz. The 18 oz stuff is used for repairing big stuff and making moulds quickly. I use 8 oz as a general purpose cloth. Maybe 2- 3 layers.

If you don't want to keep the foam in the piece, you will need aluminum or wax paper between the foam and your FG. I like to use aluminum duct tape. Or you can just use the pink foam and when you are all done, just pour some gas or something in your piece to disolve the foam.

If you use the pink foam, you will need to use epoxy resin to prevent the resin from melting the foam. If you are going to use the cheap polyester resin, then you need to use special foam -- or the green florists foam to prevent it from melting

Chopped strand mat is suitable for application by hand lay-up, reinforce and machine FRP molding, such as interior decoration of vehicles, boat hulls, sanitary ware, anticorrosive pipes, tanks, building materials, tables, chairs and all kinds of composite FRP products. Chopped Strand Mat is wound onto a paper tube which has an inside diameter of 80mm. The roll outside diameter is approximately 275mm. Each roll is wrapped up in plastic film and then packed in a carton. The rolls are stacked horizontally or vertically onto pallets. All pallets are stretch wrapped and strapped to maintain stability during transport.

you need ti build the form out of plywood or foam then lay the resin soaked fabric over it 3- 4 coats then brush more rein on then sand and paint you can use steel or wood to attach it to your car then screw it down