The same things that have always been suggested and proven to work.
Cap on the tow vehicle if it's a pickup. The closer it matches the trailer height the better.
Boattail that isn't angled too steep to be effective.
Minimized gap between tower and trailer.
Radiussed leading edges on trailer, keeping in mind that more radius than necessary has the effect of enlarging the gap tween tower and trailer, and taking interior volume away. I forget the formula for calcing radius size but it's here somewhere.
That tail just has the appearance of being so trunciated that it would mostly exist well within the turbulent zone, thus affecting flow how? Perhaps there is some benefit from it's side panels allowing some reduction in trailing wake but really the 3-4 mpg claim mystifies me.
