3) Park your X in a level spot
Speaking from experience as a truck driver, follow Johann's advice to put a twist in the straps. That buzzing in the wind is super annoying but it will also tear up the straps eventually.
I've carried two ocean kayaks on my X several times using cinch straps along with a bow and stern line tying them off front and rear just to be extra safe. Caution about using cinch straps: If you tighten them too much you can damage the hull of your boat! So don't over tighten Also be careful unloading your boat/s. We had one of our kayaks slide off the side while we were unloading once, boat was fine but my X ended up with some scratches.