To sling one canoe, hang it vertically on a long-line, and tie a tree branch ONTO the canoe to act as a tail so it doesn't spin. (Tying a tree branch or tire etc. onto a rope on the bottom won't stop it from spinning).
For four canoes, tie the ugliest creation you can so it won't start flying by itself, brief the customer that it will be a slow trip, and plan your fuel accordingly.
This stuff is not an exact science, there are no rules, just be safe and you'll have a great story to tell, (and post a photo here when it's all over).