I put a flat diffuser plate under the rear axle and fuel tank. It seems to have helped (I didn't do ABBA testing, but then again, I didn't break the 100 mpg @ 55 mph barrier until I added this diffuser) but with the sharp 6 degree edge where it hits the cockpit, and the hole for the diff to droop through, it's still pretty turbulent. See for yourself.
However, air never stays laminar after it's been drug under a car, and I think there will be diminishing returns from here on out. Do you think a curved diffuser with a gentle transition from the horizontal belly of the cockpit would be worth the effort?