Maybe this might be of interest. I have family out in California and the state is now requiring self closing vents on houses near wildland areas. There is an all metal self closing vent made for this new building code. After a few seconds of a certain temperature it snaps shut and locks. It is made by Vivico and sounds like it is the right size.
There is also another company making a honeycomb material that melts and shuts off airflow, but it cannot take direct flame. Also, it cannot be reused once it melts. It is made by Vulcan. The Vulcan product is designed to stop embers from flowing on the wind into your eaves or crawl space, etc. The Vivico product is new and sounds like it has more applications like you're talking about. It can also be reset after activation and can be closed manually. They are still testing it for fire rating, but it meets the new California building code for embers now.
Hope this helps. By the way, good idea closing off the house like that. I never would have thought of that. I'm thinking about that now.