I'm not finished with the belly-pan yet I've just received an 8ft x 4ft abs plastic sheet so I'm gonna re-do the whole undertray. Do you mean at the rear of the car? If you mean a diffuser or anything then no I'm not planning anything like that, however I might fit a couple of side pieces to the back bumper like the Prius has:
That is... if my modelling in CFD proves it's a good idea for my car.