Thanks dood.
So, got the rough build done. I need to tweak it a bit more before I call it done, but it should be doing its thing functionally. Finished it in the dark, late last night. I think the side fins are upside down.... doh. Speaking of fins, would anyone advocate putting a few more in?
The airflow under the car could still tumble quite a bit around the rear axle, but is pretty smooth from the spare tire well back. I think next would be some larger sheet metal (for heat resistance) panels that cover the middle of the car and rear axle.