I'm pondering things. One probable one is the CR-Z's engine under tray. I hear it's a near match. It covers back past the oil pan.
I'd love the Fit EV's wraparound rear spoiler, but the only one I've found is for sale in Japan for $500!
The rear wheels are set pretty far into the wheel wells, making skirts a possibility. At least I could do Phil Knox style front-and-top partial skirts.
Then again, maybe I'll just plan to stay off the highway.