Take a look at this picture from Permalink #4
The dust in the air shows your areas of concern. The dust suspended abaft the top edge is heavier than the sides because the step is 2-3ft instead of 6-8". The dust over the cab is similar to the cap you have already.
I'd make a bullnose cap that extends above the windshield as a visor. I'd also make it clamshell open reverse-alligator style to make a roof-top tent, but that is optional.
For the sides, what is the dimension of the front face of the van, the depth of the door pillar and the distance from the van edge to the opened door's edge?
In the rear, would a box cavity (Trailer tail™) enclose the sport bike?