I use a Bell 2 bike trunk mount carrier. Besides my relatively level top bar bikes, it handles my wife's cruiser, and mountain bike, both of which have odd configurations of top tube, I just move them around so they fit.

I modified the carrier slightly by adding side straps for extra stability, and black neoprene pipe insulation (self sealing) for where the carrier can come in contact with the bike frame. Like one of the other posters, I just use a couple of bungees to keep the bike stable, and to stop the wheels spinning. If I have 2 bikes on there I use a cam buckle strap to anchor them both in place to the carrier, and with the side anchoring straps, the carrier doesn't move at all even when it's fully loaded with a couple of steel frame cruisers.
Takes me less than 5 minutes to get the carrier mounted and the bike on.