This is my setup. The roof racks came with the trailer.
I used 1 inch X 1/8 inch (or maybe 3/16) flatstock cut 2 inches long. Each corner takes 2 pieces, one inside and one outside.
The inside piece was drilled so a 1/4 X 1 1/2 carriage bolt sat flush and was welded in place to make a T bolt. The outside piece was drilled to clear the T bolt. A poly locknut was installed to clamp it to the rails.
I drilled holes in the back side of the solar panel frames to match the roof rail spacing.
Then on the T bolt a reg nut & flat washer was installed, the solar panel, another flat washer, then a poly locknut just to where it was flush on the back of the nut.
The whole assembly was slid into the rails and tightened.
All hardware is stainless steel.
The panels are connected in parallel and the feed to the controller was routed around the side, up through the roof/wall gap and down to the controller.