Considering the lack of tie-down places that are available on most modern cars, and not knowing what sort of situation you are faced with in that regard, I’d recommend renting or borrowing a utility trailer and strapping the canoe over the top of the open trailer box. Maybe you can do a good job of tying the canoe to your car’s roof, but from what I see on today’s cars, the odds are against it.
If you do put the canoe on your roof supported by foam blocks, make sure all your tie-downs are angled in a way such that “all of them are fighting each other.” That means that you can’t pull a single one of those ropes tighter without adding tension to all the others. At a minimum, you will need two in the front and two in the back, with each pair diverging at a wide angle. And to achieve the state of “all tie-downs fighting each other”, you probably do NOT want the rear pair of tie-downs to attach to the end of the boat, unless that’s just an auxiliary pair. The main pair in back should instead attach to a thwart, so that the lines run forward from the rear of the car on their way to the boat. That would be good idea in the front too, to increase the degree to which one pair of forward lines “pull the boat forward” and thus increase the degree to which they oppose those lines in back. Oh, and related to all that, do NOT just loop ropes over the canoe and down the other side. Instead, use a pair of ropes, each attached to a solid connection point like where a thwart joins the gunwale. You don’t want to encourage the boat to act like a pulley within a V-shaped loop of rope, and be free to wander back and forth.
I’ve seen the way too many people do this in really sloppy fashion, so I say seriously, putting a canoe on the roof with nothing but foam blocks is a recipe for having the boat wander all around up there, unless you’ve got quite a set of ropes aligned in opposition to each other, so again, if you aren’t pretty good with rope as it is, I’d recommend the trailer option. At least in that case you’ve got a solid support system and the ability to anchor the boat directly to the object that supports it, instead of to anchor points that are really far away from the boat. Are you worried about the cost of renting a trailer? Think of it as a very low purchase price for your new boat!