I think you don't have to cover much of the gap between the truck and trailer. I towed a teardrop trailer behind a '99 Montero (both fully loaded) and barely felt the weight (except up hills) I figure the reason was that the teardrop trailer actually helped the montero close the air behind and the size of the two were fairly even. You probably can't do much to fair the tail of the U Haul but the truck could have a topper slightly higher and wider than the trailer. I suppose that the air leaving the back of the topper would follow much the same pattern as the aero template (teardrop) as it attempts to close behind the truck. So as long as the trailer fits just inside this closing air it would ride smoothly in the wake...and then create it's own horrible drag behind it. So if you draw the aerodynamic template over the topper and raise it to cover the gap you can imagine how the air would flow. A larger image would help, but this little sketch looks like raising the topper about 8 inches or so above the height of the trailer would be about right. A flat plywood roof would suffice. I don't think you gain much by curving the topper. You will gain a tiny bit by smoothing the plywood and painting.
Note that my image is not intended to suggest that this is a perfect aerodynamic shape, merely show the path of air leaving the truck topper to meet up with the upper edge of the trailer.
