I bought a trunk frame that stands on the bumper with two legs, and two hooks with straps to put on the upper-side of the back door or the backside of the trunk barge.
works pretty nice, with two extra straps to secure the C5 firmly to this frame. Original it is intended to carry three bikes where the two hooks that stick out carry the frames.
Does it make any sense, what I tell here?
And of-course I have a roof-rack, that works as good as well.
greetz
Frank