When I had a car with gutters like Foolestroupe's I made a roofrack box like his, too. My box was ply, with joints reinforced with alloy angle. The front was wedge-shaped for downforce, and the sides were pierced for tie-downs, and there was no back so that it could take longer loads. It had no noticable effect on fuel consumption or top speed, probably due to the shape.
It was considerably norrower than the car, because I once balanced a car on two wheels through a long sweeping bend, inching ever-nearer the centreline. That's what happens when you have an overloaded roofrack and are cornering too fast.

I got away with it that time, but I'll never forget it!