I have a Thule V-rack that mounts on a Thule or other square bar roof rack. If you are not familiar with it, it mounts the front fork via the dropouts and the rear is held by the axle (with the wheel mounted) via its own quick release. It is, of course, a 2 person job to get the bike up on the roof, but is substantial and reliable.
You can have it for $150 plus shipping.