Heres what I came up with.
It follows the bumper pretty much straight down. I'm not worried about overheating, I drove up a 5% grade at 80 mph in 90 degree weather with the AC on full and it didnt move.
The 'dam' goes to about an inch below the lowest part of the vehicle, and I've still got a decent amount of ground clearance and managable approach angle. The worst that can happen is I lose some time, all the materials have been free.
I wish I had a scan gauge or some way to test this.