I have been brainstorming this roof rack idea also. Have found out schedule 40 2" pvc just fits inside the 2 1/2 " with a few thousands of an inch to spare. By using tee fittings you could build the rack out of the
2 1/2 . By using the same tees put on 4 feet whereever you have support (spars) to support and attach the rack with closet connections. The 2" u shaped loop would slid into the front and back rail and telescope out when needed. It can be removed and refitted to the opposite side if need be. With the new plastic paints it could be finished to match or contrast as you desire.
I had mixed feelings while needing the extra toteing space I didn't want to spoil the view of the night stars thru my lexan roof by stareing at the underside of a roof rack but you guys just came thru for me because what I hadn't thought of was drilling thru the side wall and supporting the 2 1/2" tubes off of the underside of my spars for the 2" extendable curtain rod
UNTIL NOW ! 