While I've never loaded either of my canoes on my truck yet(I've got a boat-loader,I wonder how that's going to work?),I've sure done some distances with them on my Cherokee. Make sure it's solid,gravel road will loosen almost any, except quality straps.I used simple clam clamps from Clipper.For under the gunnels on the Thule rack I slide on heavy bike tube.Over the canoe,under the strap, that waffle rubber used in kitchen shelves. At the front and back as wide A-frame tie as you can do. I put stainless eye-bolt on the undersides of the bumpers.
