I have a Riken 14 expedition style raft and they are super skinny. My 85 qt Yeti BARELY fits between the tubes, and most definately rubs on the rubber on each side. I haven't noticed any wear on the tubes and just go with it. I figure, it's designed to run into rocks. My smooth plastic cooler ain't gonna hurt it. Would I do this for twenty years? Probably not. Why not put some astroglide personal lube between the tubes and the cooler? Unless the smell gives you a chubby.....
Dude, I'd see you on the river but I'm hardly ever out there.