I've seen guys use two PCV pipes with rope pulled thru them with a small loop ate one end of both. They then ran a separate rope or bungee cord around the columns, then thru the loop. The other end of the rope then is tied to a cleat. , front and rear. The poles limit the boat from being able to get closer to the column than the pole is long. Protects the boat and keeps the boat in a stationary position. I never tie up, but this seems to serve the purpose and is inexpensive.