If you're still planning on doing this, you might want to skip the computer model altogether, and go straight to a mock-up on your actual car. Here's a couple of links on Youtube I saw, and was planning to make use of myself sometime.
This first video may already have everything in it you need to know. Creation of a fully custom rear spoiler for an Audi A6, production mold and all:
The second video is by an inventive Australian guy who does some pretty "out-there" designs completely from scratch. It might look rough to start, but the idea behind it all is sound:
I think that even the most accurate computer model will still have to be tweaked once you see your new design on a real car. Going straight to a clay mock-up might save you time in the end, and get you moving on your project now instead of waiting on perfection. Pay close attention to the second video when it mentions the use of templates. I'm not sure his method is the most efficient way; but even with the most inaccurate computer model, you might be able to print out a set of scaled cross section templates and use them as guides. I'd love to see what you come up with. You can do it. Just start