I added a curt basket to my car and while testing my F/E "hit" that I expected I actually saw a gain!
It's this model
CURT Manufacturing | CURT Manufacturing - CURT Cargo Carrier #18140.
I choose this one because it curves up a bit to get it out of the air flow and it also adjusts front to back to get it as close to the bumper as possible. It's still very easy to load the trunk. I also got it at about half the price listed there.
I got about .5l/100 better of 6.1 to 5.6. I did 3 runs to and from work with it, then took it off and saw my tank avg increase after 3 more runs to and from work. Put it back on for the rest of the tank. Next tank took it off for the first 3 runs then put it on and saw the avg drop. It's been on every since. In the summer I may play with smoothing out the bottom and sides with coroplast and using plati dip to stop the sun drying it out. I won't touch the top though so that I can still use it for it's intended purpose. It really came in handy when I picked up the xmas tree, so far the only time it's been used for it's original purpose.
One downside is most of the rubermaid boxes I see are listed as about 1in too wide to fit, however they mostly taper so I'm hoping that when I actually try one it will fit down at the base.