My Dad used one years ago when we went on long trips. The frame work is made of light weight angle iron. It has 4 'feet' with suction cups on them. The rack is secured with metal clips that hooked onto the edge of the drip rail that had straps that went to the framework to tighten it down. I think the suction cups were to prevent movement once secured and to prevent marring the top.

It worked fine although it's possible the clips could have done some damage to the drip rail molding. I still have it in my garage but the clips and straps have gotten lost over the years and the suction cups have gotten hard.
Dallas