Great Stuff doesn't look ideal, since it's going to get a lot of sunlight on the outside, and Duct Seal doesn't say anything about whether it actually insulates.
Also, the Duct Seal material safety sheet doesn't really convince me that it's %100 non-toxic to inhale, and I am somewhat sensitive to chemicals. I'd like a really green alternative. Mycelia?
Note--this is a different situation from the question "How should I fill a hole in an exterior wall?" since I have no cables running through the hole and simply want the hole filled and insulated. I live in a cold climate. Thanks much.