panelling under axle/ rear diffuser

[vmax1974]

The rear defuser on the saturn car was only designed to stop the rear panel acting like an airbrake not to give any downforce

If you wanted downforce you would have to taper the fins from narrow at the front and wide at the rear with no lip between the defuser and the rear panel

You would also need to add a lip to the front of the defuser to start the airflow circulating towards the defuser

The fins then speed up the air passing through them creating a vacuum which pulls the car towards the floor
But if you are going to go this far you should also think about unloading the pressure build up in the wings by venting

And maybe disturbing the airflow going over and through the front wings to help reduce the oversteer

Just my thoughts