In an emergency, a 2x4 on the inside of the canoe, thin ply on the outside, and you are good to go. Catamaran is the safest. Lash the canoes just far enough apart that you can pass a beer from one canoe to the other with a paddle. Any closer and you get a standing wave that will fill one or both canoes. Farther apart and you need to have the beer in your canoe.
Side mounts are ok, but really throw off the balance of the canoe, but you won't get as wet as with the 2x4.
