In a pinch, you could also get one of the trunk-mount carriers, too. They have rubber or foam pads so as not to rub the finish. I prefer the rooftop mounts personally, but I'm sure the trunk mounts work fine. I see them frequently enough!

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1992 300D 2.5T
1980 Euro 300D (sadly, sold)
1998 Jetta TDI, 132K "Rudy"
1974 Triumph TR6
1999 Saab 9-5 wagon (wife's)