Product review (5 stars): It stops the flue fer sure!
This works just fine.  It has a nice finish to it and is large enough to completely cover the hole and the crumbling mortar around. It has a piece of bendable metal in back to fit it to the flue, which a bit different from the spring-loaded type I have seen before, but it works just fine as long as you don't bend it too much.  Admittedly, is a bit drab, so we used some acrylic paint to add a cute design that fit the d&eacute;cor of our lake cabin in Minnesota (see photo of decorated flue stop)