The following link is from the RoadRaceAutoX forum, which is dedicated to car racing on road courses and autocrossing. Please note that much of the design is geared to producing downforce or reducing lift and improving cooling of the engine and brakes. However, the cars must use all of the stock bodywork pieces and other modifications are very limited as this is amateur racing with SCCA or NASA so this is all being done on a budget (no carbon fiber to be found here...)
Many of the cars are Honda CRXs & Civics, Nissan Sentras, etc. so the construction techniques and materials can apply to any car on the road.
Splitter Design - RoadRaceAutoX
This last image is of the racecar that I now own:
I hope some of you find the information useful.
Tim