I use Down East S-10 clamp-on rod holders on the gunnel. They are a bit heavy (maybe 20 oz. each), and they rattle a bit when you take the rod out, but they are bullet-proof. I would not hesitate to use one on a rental canoe. I put a small piece of wood behind the clamp to stiffen it up. My partner did not use the wood bracing, and it also worked, though we both thought the bracing was better.
You can see my bracing in the picture.