trunk racks are cheap...
as for the roof idea... I'd go for flipping the bike upside down so the handlebars + saddle rest on the roof. Then run a ratchet strap through the frame, and through the windows of the car making a loop.
I personally wouldn't risk doing this with my bikes on a roof. That's just how I haul my bikes 'round in the bed of my truck and i don't have any problems...