www.nudo.com/p_polymetal.php
https://www.grimco.com/Products.aspx?cid=0198
http://www.alumapanel.com/subcategories.cfm?lines_ID=3
I worked with scraps of PolyMetal and a hand shear/brake/roller. Larger pieces would require different tools, like an edge roller. I was able to shear and roll foot-wide pieces by hand at an 18" lever arm. Braking the angle ruptured the outer aluminum skin, which didn't affect my demonstration. Scoring or a shallow cut would relieve that. I'm not sure what tool would score it—short of a broad chisel and hammer, but remove one skin and about half the thickness of the plastic would be about right. Then you could glue or solder the inner edge.
I like the Roswell. There's a way to make yurt-like pods of thin plywood like that. I adapted the idea to The Template that is so popular around these parts.
This actually follows The Template at very low resolution. The front angle is 45° and the rear (if the truncation were extended) is 45°. Close enough to the theoretical ideal of 44 1/2°, which number came from a wind tunnel and I'm convinced is a measurement error.
Anyway, scaled up for your purposes, it might have a longer center section (with internal wheelwells) and a shorter truncation to fit a bigger back door. As you shrink the trailer around the car, it will get harder to get it in and out. If the width 3' above the floor is the widest part that might help. If I was doing this, I'd want a 2-piece floor with the front section having arms that reach back to a pivot line in the middle of the back section. Then you could open the door[s] and drop the back of the floor to the ground. I guess it would be like a roll-back truck. With an automated winch so you don't have to drive the car in and then open the door to get out.