The only thing that I plan on putting on top of the van are solar panels. Roof racks create a lot of drag and are expensive so building one yourself not unreasonable. My idea is to have angle aluminum down the side. 1x3, 1x4 whatever. The angle would be high enough to cover the thickness of the panel and the arc of the roof. Mounting clips welded on the inside of the angle to mount the panels. On the front some type of air foil. Powder coat and bolt it on.