Yeah, that adapter is the kind of thing I had in mind (sans thumbscrews... looks like the guy spent some serious time on that bad boy
You wouldn't even need a rubber pad, Mercedes supplies that on the car.
Another option might be to make a little steel box, closed on all but one side, that slipped over the top of the support saddle. No tricky machining required to fit the countour of the saddle. Though I suppose the weight of the car would then be supported by the outer edges of the saddle which isn't ideal.