After the foam they glazed it with fiberglass and some resin or something.
I am wondering how it would work if you were to use one of those Mach 1 chin spoilers to build on! Also I wonder if you could use some of the plastic honey comb behind the spot where the bumper lens/side marker goes, load it up with foam in the front and back, shave it down, fiberglass/resin it up and sand it, prep it for paint, and have it shot with some color rather than just paint over the things.
I think I am going to use this site to make a roof spoiler also.
Cut out a sheet of foam to fit the top of the rear windshield, layer it up with some weaved cloth, fiberglass it and shave it down.