Use the duct putty to 'hold' 2 pieces of plastic together, to each other??? I guess, maybe, if the plastic was layed horizontally but this is installed vertically. How would he do this?
Certainly the putty has the properties you described. I'm just not quite sure how you're installing this??? I could use a new idea. Usually, something is pressing up against the plastic to hold it in place. Or I just don't worry about it. If water wants to come in it will come in. But I don't worry about the tiny bit of moisture that might come in.