I put the 13 foot sportspal on the roof rack and the gunnels were just a little bit wider then the top curve of the cross bar. It worked but next time I am going to put two 48 inch 2x4 tied to the cross bars to stick out 4 inches on each side beyond the rails to make loading, unloading, and final tie down easier.
For what it is worth, i used three rachet cargo straps with s hooks to go through the side rails of the roof rack. Went over the canoe, around the siderail, and then back over the top of the canoe again. I have found that if you go around the canoe under it, the unsupported strap makes a terrible vibration and noise such that over top twice is the way to go. 12 foot long ratchet straps were long enough. S hooks to fish them through the siderails.
I see a lot of cargo with two straps going down the road, be it canoes on top of vehicles, trailers, whatever. I always use three straps in case one strap fails for one reason or another. If a strap fails, i might get some vehicle damage from the flailing strap but my cargo is going to remain until I can get off the side of the road safely. In a two strap system, one strap fails and your cargo could fly off and kill somebody.