Just got back from a long road trip using a trunk-mounted bike rack
(originally purchased for my Jetta).   The key to making it safe was
to add two adjustable bungie cords for lateral (side to side)
stability.  I found that the hooks would fit nicely in the air vents
on the back sides, and the other end of the adjustable cord would
clip directly to the bar that the bikes rest on.  Without those
cords, the whole thing would move left & right too much for my
comfort.

Also, those racks have a tendency to unclip the bottom straps if you
hit a bump (the rack shifts down, and the strap loses tension and the
clip slips down off the bottom of the door).  I put a bungie cord on
the two bottom straps as well to keep them under tension.