[beluga]

Got my best ever MPG last week after a bunch of tiny mods, ditching weight and massive P&G (ankle hurts, can't do toooo much). 450 miles / 12.7gal = 35.433070866141732283464566929134 miles per gallon. less than the xl1, but good for this boxy thing.

i think the next main mod must be a kammback wing, somewhere between the partial metrotail and the pickup truck down-wing i saw on here a while ago.
other kammwings i see that seem like good models (of course they have the whole wing hump profile up front, too, which mine doesn't yet)---
honda fit
prius vee
prius see

valence, wing, then belly pans and pizza pans, in that order seems most productive.