-- Last Updated: Sep-26-12 7:09 PM EST --
The foam blocks I was thinking of are the kind that slip over a crossbar on a factory roof rack. I was not envisioning the kind that just sit on the roof of a car without any crossbars (which are not what I think of as a rack since we always go third party).
I have another idea then. Still take advantage of the thwarts, only use both of them. Run ropes or straps thru the thwarts and fully around into the inside of the car, tie off there. Place knots or similarly useful stops at appropriate points to stop the canoe from sliding sideways from the original orientation.
No guarantee that the whole assembly can't slide, but it will take more effort for it to do so.