Any pictures?
I've never done this, but a boat-tail like back end would reduce drag on your vehicle the most. You certainly have this option since you do not have a bed, however, you would probably be sacrificing aesthetics in the name of economy in this case.
Do you do mostly highway driving? Aero mods will benefit you the most if you do a lot of highway driving.
There's also a page on here you can check out to improve your driving habits. I believe you could get up to a 30% improvement in mileage by improving your driving.
https://ecomodder.com/forum/EM-hyper...driving.php#71