I think that the first step would be to simply extent the undertray behind the rear wheels then build a vertical air dam above the undertray behind the rear wheels to keep most of the dirt out, build and mount boat tails behind the wheels on the underside of the undertray. I suspect that it will take two or three versions befor you get what you want,and what has the best aerodynamic affect. There are as many ways to do this as there are people doing it, have some fun and see what works for you.
