Hey, you can get racks that clamp onto the groove at the side just as the roof curves round and press against the curve next to the gutter rail on the outside. Problem seems to be that it leaves dents in the roof. At leas, mind did when I took it off.
They are strong and work well (genuine VW T4 accessory catalogue option), but sadly just where that picture is taken I had a small area of rust appear on the roof above the leading edge of the sliding door. I removd them some time ago, but have been considering getting another pair to mount an awning (again, as I do not want to drill the roof) T: