Nice cover, ijames! My dad has a 2002 2500HD crew cab, short bed with a Duramax and 4wd (I believe Allison tranny as well), he gets about 1MPG less than your stock numbers. He isn't interested in doing stuff like skirts and belly pan, but he likes your cover. It looks like it wouldn't be much trouble to add folding legs to it to make it pull double duty as a table.
I put a pan on the bottom of the lower control arm on the driver's side today. I have a picture of the bare passenger side for comparison. Passenger side is cut out, just needs installed.
Took another trip to Home Depot, averaged 15.7MPG compared to 16.5 for the coroplast trip. Traffic was pretty bad (El Paso drivers are terrrrible), got stuck at a couple lights, and had AC on for the return leg, so really not that bad. Tank MPG so far is looking a bit better than I would usually expect.
My wife has banned me from doing any more aeromods til I fix the climate control. It blows hot air out of the passenger side (dual zone system) regardless of the settings. This results in warm air seeping through the closed vents and the driver's side air being shared, which is why the AC was needed on today's trip. Pretty sure it's a bad blend door, only need to take apart most of the dash to get to it...