I was in the same boat. I wish I could have found a roof-rack in my price range. You already ruuled it out, but there are some major advantages to a portable trunk unit.

1) You can remove it, keeping the clean lines of your car the way you intended.

2) Its cheaper. Money may not be an option, but if your like me, an extra hundred bucks can go towards some car mods.

3) It will most likely not scratch your car. If your worried, you could always use some 3m clear tape (the ones people have been putting on their front bumpers). You could put a clear strip along teh edges where the rack would attach. While your at it, throw some on the front bumper and protect that as well.

Just a few things to consider.