You got the wrong vehicle - you need something like a first gen Miata with a roll bar to transport a canoe...
But seriously - you need to secure the front from lifting up, catching wind, and launching itself off the car. Definitely thinking you need a roof rack to distribute the weight to areas that won't crush when strapped down with force. The canoe may only weigh 50 pounds, but you gotta secure it with more than 50lb of force, plus whatever forces the wind applies at speed. Could be hundreds of pounds.
Is there a tow hook on left and right sides in front? You could maybe secure it to both and triangulate the force to also stabilize left/right movement. In the rear you might find a spot to tie down or else improvise something as long as you're not crushing the plastic bumper cover with a tightened rope.
Overall, this will be a bit of a challenge. Risk to car damage is great. Maybe take up kayaking instead...