CLC’s prototype has leading and trailing edge fairings to reduce the drag. The other thing to keep in mind is the angle of attack. If the panel slopes toward the back it’ll be generating lift, as well as drag, and will try really hard to fly off the roof. Sloping forward it’ll be getting pushed down onto the racks.
For absolute best performance, you’d want to completely do without the racks while driving. They disturb the clean airflow over the top of the camper much more than the panel. You’d also want the panel centered or have 2 panels spaced symmetrically on each side of the overhead fan if you are keeping the rack. For that matter, the tongue box can also mess up the airflow unless it’s in the wind shadow of the tow vehicle.
As a practical matter, though, it doesn’t make that much difference, especially if you drive at reasonable speeds, so just do what’s convenient for you.