I'm not sure glass is the "right way" to do it. Andrew's plastic was definitely way easier, and it looks fine to my eyes. The only reason I wanted to use fiberglass was so I could use the foam base to get good compound curves on the sides/"shoulders" which I ended up not getting anyway when I abandoned the foam for cardboard.
That said, I got two layers of glass on the outside today.
Tomorrow I'm going to make a "form" to hold the shape of the trailing edge, then remove the Kammback from the car to resin & reinforce the inside.
After that, I'll come up with a mounting approach, and then start on the finishing work (bondo, sand, sand, sand, sand... Whoops - thanks for the bondo tip.)