Posted down in the Freshwater section, but will post it here too. All we ever used was two 3/8" thick pieces of plywood, roughly 4"x8" that were connected by a couple short pieces of webbing. The webbing draped over the gunnel and one piece of plywood went on the outside and one on the inside. The 2 hp Evinrude outboard then clamped on the plywood and "sandwiched" the canoe wall. It added strength and prevented damage, but was light and easy to take off if you wanted to paddle instead. Probably cost less than $10 and took less than an hour to make.

SS

"To be glad of life because it gives you a chance to love and to work and to play and to look up at the stars. To be satisfied with your possessions but not content with yourself until you have made the best of them."
-Henry Van Dyke