HAving a 4-day weekend to play with some mods, and having read the thoughts of you guys who have much more experience with such things... I went to Lowe's and bought some composite garden edging to make mini wheel dams with. I'm going to bolt it to the underbody such that it can deflect some air around the front wheels and suspension. It will NOT be quite so low as my elegant but impractical first attempt
I also got a sheet of coroplast to fab up the beginning of an underbody tray, starting at the bottom of the front bumper. Pics to follow.
I appreciate everyone who has given input so far, I think it'll help me avoid some mistakes that I would otherwise make on my own...