DO IT!!
i've wanted to for years, on my grey one (the one in the registry)
of course, you're gonna hafta cut off the rebar and rear (steel) bumper
somehow, you'd have to weld a new (probably flat-bar) rear bumper under the car (where the original was, only back farther), so if you're ever in a crash, the vehicle doesn't smash like an accordian (and i've seen cavalier wagons in rear end crashes... not too happy with that one . . .)
but, after that, molding a custom metal rollpan wouldn't be all that hard
to accomplish
it'd just take quite a bit of time