I put my bu-353 on top of the dash like you showed in the first pic.
I saw a post before by justintime (click link) that shows putting it under the roof rack molding. I'm guessing it would work fine in your rear spoiler. Unless it's so thick that it blocks satalite signals. My only thought would be how you'd connect it from the spoiler to your computer, my usb cable is only like 4 feet long. I'm sure you could lengthen it somehow though... good luck with it