If you can, extend the sides further down over the wheels - say down to the lower level of the toolbox - angling up aft of the wheels for tipping clearance.
The front of the box could do with more rounded corners - like the tube on the top.
It's probably better than a squarish slab, even if pushes some air in the gap between cab and dumpster.
A tube cut lengthwise in 4 quarter-circle "pies" (like the crosshairs of a gun sight) could do the trick with decent radius yet while still being narrow enough to clear the cab. Fill in the "open" side with plain sheeting.
I wonder what a tank @ 45mph without AC would be like
