Ok, here we go.
The first photo is standing right behind the truck. Note the line of the curve as compared to the line of the cab. The curve is the intersection of 1" inside the contour of the cab, give or take a quarter inch or so. This means I can maintain the edge contour from front to back, which I think is really clean.
Ford did a lot of work on the airflow over the top and side of the truck, so why ignore all their effort?
The second photo shows looking straight up the curve, so show how it and the hinged section will angle down, making the lid a trapezoid.
The third photo is a clearer shot of the curve from the side.