Fiberglass molds can be a lot for work but very rewarding. I have done a few and you learn quickly that it is very much about the details. The mock up part has to have a mirror finish like you could use it on the car. You cannot have pin holes, blemishes, it must have a mirror finish. I paint mine with single stage enamel. Here is one I did for an intake duct:
Basic shape with formers:
Fileld in and shaped with foam; in this case expandable foam. I have also used balsa wood; plywood for vacuum formed molds:
Fiber-glassed, finish glass smooth, painted with automotive paint:
This one laid up in 2-part mold on an old aircraft window. Glass is good too for separation:
This is a great web site for mold building. They not only have supplies but great how to videos and books. Here is an online tutorial. This can be a very time consuming project. It is all about the details!!