This arm pushes the door against a thick weatherstrip inside the 8x8, a limit switch stops it once it is tight. It took me about half a day to build it but it is very much worth it. There is no air leakage and the door has polyiso on one side and is very well insulated.
Great info, does the damper require a particular height during the install that makes it more affective? I've been looking for videos on how far up the pipe the damper is best situated, and I cannot find any information about it.
When I needed this a month ago I didn't like any of the solutions that are available. Unless you go for laboratory grade of course which is very expensive. I just built a 8"x 8" box out of treaded 2x12's, a foot long, and at the end (inside the building) I installed a 7 1/2" x 7 1/2" door, on a small piano hinge, that is moved by an electric arm (they are available all over, I took mine off of a commercial air line) which is activated by a thermostat.
