The Mitsubishi Evo and Subaru STI have functional vortex generators on their roofs, because they act to put the giant park-bench wings into airflow, making them work better at generating downforce (and drag).
It's been posted elsewhere here, but Mitsubishi put out a paper on their VG design.
You can probably buy ricer-ready knockoffs without half-trying, or spend the money on the real thing from Mitsubishi or Subaru (and have them probably be nearly as ineffective as the cheap knockoffs, because your car wasn't designed for them).
Google shopping results for 'evo vortex generator'
evo vortex generator - Google Product Search