[j-c-c]

On my 2004 Tacoma, I did something very similar. I was not looking for max mileage over functionality. Regardless the results surprised me. I wanted to retain rear window useful visibility via the center mirror, and I wanted the rear sliding cab window to remain accessible, for the occasional transport of say a long pipe. Built a 2x10 sided wooden box, with a top flat section of 1/2" Baltic birch PW approx 60" wide x approx 42" long, At the 42" point I straight tapered the 2x12's down to approx 1"h, and over hanging the tailgate approx 3", and then hinged another piece of Baltic birch PW. This overhang had a 2x2 near the end L to R which effectively locked the tail gate from being opened without lifting this rear hatch. I made removable legs so I could securely prop up level the rear hinge hatch to haul items as needed on the 84" combined flat surface, which I reinforced to handle loads I expected to carry. I installed locking pins to secure the rear hatch, I turnbuckled the entire assembly to inside the pickup bed for easy removal.
This truck has currently 550,000 miles on it, it has a long record of expected fuel mileage. First trip with the box/hatch assembly of 3,000 miles, mileage increased from 22 to approx 23.8, even with the approx 100+lbs of extra weight. When I returned, I did a wool test to try understand what was really going on, and found out just by luck/gut, I had the hinge point/taper start right where it seems airflow began to reattach after flowing over truck cab/roof. Not sure what part the tailgate hatch overhang/lip played in this entire experiment. And to enhance usefulness, on both sides I attached externally to the 2x10, I secured alum aero tie down snap rails to increase flexibility.