I would go for the Styrofoam!!
The stuff is easy to cut/glue/form/sand/paint and so on.
Go for panels that are 1.5 inch thick. I think cost is around $15 per sheet.
After the shape is complete, add one layer of 6 ounce fiberglass cloth and epoxy to the foam, using the wet layup method. Make sure to add about 3 layers on all your hard-mount points, and embed wood blocks in the foam in these areas.
For a boat tail, you will probably go through about one gallon of resin, and if you use West Systems, it will set you back about $100/gallon. Cloth, about $40 or so.
The cool thing about foam is, after your done, it will look just like an automotive finish. Of course don't ask about how much time the sanding/glazing/priming and painting will take. But this is something you can do anytime you wish. Just take them off the car and start sanding. And you can still drive the car in between.
Just my $.02, Jim.