Earlier this spring, I noticed on an Insight that Honda had added some devices under the rear bumper cover when they refreshed it for 2012:
I found them in a parts catalog--they're called "rear air splitters"--and ordered a pair. The Prius' bumper cover is slightly too short, but the locating pin at the end of the splitter still fits in a hole I drilled in the cover, so the whole thing is stable.
This will be the last mod I get to before I do coast down testing; my anemometer came in today and the wind direction vane should be here tomorrow.