Embrace the box—and then think outside it. These exist, or you could make your own.
As chillsworld, says, boattail it. Here's a sketch I did for my VW van. It has a vertical tail in the back and an horizontal wedge in the front to lower tha stagnation point.
You could hinge the tail at the top and have power jacks. Then it would open up into an 8' long Gothic roof over a fold-down floor section.