A tonneau cover based idea...
Take your basic tonneau cover, as shown above...
Build an extension on the truck bed sides that slopes to the end of the bed. Also, you will need to build a "bridge" to fill in the gap to the back of the cab.
The tonneau cover can still open as before, giving access to the bed of the truck.
From the side, you can see that there is an arc built into the bridge, allowing a smooth transition.
My 3D model isn't the best, and neglects lots of things like space for the cover to open without contacting, but you get the general idea. Also I will point out that the cab in the first picture has (heavily) tapered sides, and the following ones do not. It should be easy to put windows on the sides and the portion behind the cab to allow viewing out the sides. Although I had this as an "epiphany", I know there is a very professionally done one out there somewhere that looks just like this, and I didn't even think of it until I had it drawn up and said "this looks very familiar". Does this seem feasible to folks?