The old vent hood that I just removed didn't vent to the outside; it just filtered the air and recirculated. This is how I want to keep it since I don't cook that often and when I do, it's usually just to boil water.
The microwave has a rectangular opening with a flimsy metal door over it which is the output of the vent if going to the outside (up through the top cabinet). Since I just want to re-circulate, can I just leave it as it is, or do I need to close that off more securely.
Here are pictures of the top of the microwave:
or should I just remove the metal door frame completely as shown in this pic and install it that way: