I can't answer the legality question - depends on your local/state situation.
Material choice for a partial Kammback is a tough one. I'm going to try making a fiberglass one myself. It'll be light, and perhaps easier to get the desired shape (by making a foam mockup).
Visibility may be an issue. You'll note that
Mighty Mira used clear plastic on a metal frame partly for that reason (I'm assuming):
My kardboard kammback mockup blocked the upper 1/3 of the vertical height of the rear window, and I still had a clear view of traffic on a level road behind me in the rear view mirror.
