Along with double bow/stern lines, I would tie canoe to carrier/roof rack, then take a good cam lock strap and pass thru rear seat side windows and go over canoe and snug tight.

Tie a piece of string/twine on strap where it meet the closed side window, and if it rains the water will run down string and not leach into cab.

You might lose one bow/stern strap and the rack might loosen or brake free, but with the added top strap the chances are you'll have a chance to stop before a possible tragedy happens