The easiest way I know is carving rigid foam and sanding it. Then covering it with fiberglass. If it's a one off you do not need a mold, just scrape out the foam after the fiberglass cures. I used this method to repair bumpers that huge pieces missing and broken interior panels.
what silet said
Styrene will near completely disove the foam also, so you need a barrier like foil, though its better to use an epoxy primer or use epoxy entirely as opposed to polyester resins
You can use spray foam but it does need to be filled in larger air pockets once you cut it out.
Oh and gas works to disolve foam once you do your layup of glass