On the Fabulous Operating Products SAM-1-81 Range Hood Upgrade Kit, not sure quoting a year old review by someone who obviously hadn't researched what they were buying and what would be involved to install representative of the fan.
Installation instructions for the Sam-1 can be downloaded.
http://www.fabulousoperatingproducts.com/installation
Not that I'm buying- but replacing that 'lil plastic propeller with a squirrel cage fan would be nice.
But question was about the exterior flapper door (albeit 3 years ago?). I also used our awning rod to open/close. Primarily due to the tapping noise at night. I converted to a cable push/pull setup several years ago but was a rather involved project. Awning rod simple, or 4' of Ø3/16" rod bent to suit.