I use Loop straps, top and bottom, with the cooler snug against the cross bars. No rotation of the cooler.
My 1st raft had a drop bag for the cooler and while it worked fine for holding the cooler, it was hard to get to the drain plug. I don't mean to start a whole different discussion, but when I didn't know any better, difficulty in draining didn't matter because I didn't do it. Now I know better and drain daily and like to be able to get to the drain from the side.
I also rig my cooler in front of the rowing bay because I'm getting old and want to be able to sit down when I'm getting into the cooler. I always strap a Paco pad over the top for total sun protection.
Karma is like this: If we set causes in motion the effects come back to us.