A few days ago I bought a few 3'x6' sheets of coroplast to make a belly pan with, and decided to visit my father's to make a weekend out of it. We're going to work out a way to insulate the exhaust and make some bracing. I loaded it up into my car by pinching it in the back window like so:
Normally, I can get 37MPG cruising at 65MPH, 33 at 70, and ~30 at 75. Even though the coroplast isn't flush with the roof, and isn't quite as high as it needs to be above the trunk (I still had some turbulence, should be about 3" higher I believe), I had a significant improvement!
92 miles later:
I've never had more than 37MPG on an extended trip. Weather was in the 50's and a bit wet, so probably not even ideal conditions. 37 -> 42.5 is a 15% improvement. I'm seriously considering constructing a detachable tail with feet to hold it above the truck lid for road trips now, and am excited to see what the combination of boat tail and belly pan will provide.