People, canoes, and cars are so various that it's hard to describe a system that will work for everyone. On the truck I use to carry my canoe I finally broke down and drilled holes in the ends of the bumper and mounted stainless steel eye bolts. That gives me some great tie down points to secure the canoe at the bow and stern. Along with a loop that goes around the belly of the canoe and is secured at each end to the forward shackle of the leaf springs. What ever system you come up with I would suggest that you try it out at home, put the canoe up there and drive around a bit. Let the wind and motion of the car play with your lashings and then pull over and check things over. It's always surprising to see what you thought was a bomb proof system starting to unravel, but it can then be improved with out much strain. When I'm on the road for real I do the same thing at every rest stop, just for insurance.
I have a good old truck that is pretty much dedicated to canoe camping, I wish I knew of a paint shop that specialized in making a truck look like a leprous piece of junk. Something that no one would covet. In fact so bad looking folks would park far away just on the off chance it was catching!
Best Wishes, Rob