Years ago I had one for our Triumph Stag for just that reason. It had suction cups and straps with rubber coated clips to hold it on. My only worry would be if the rack would dent the aluminium trunk. I just saw on the Hyman site out of St Louis a 280SL with a trunk rack permontly mounted.
Bruce; 268Blaugrun(green) 1970 280SL; IL