Hope this is a good forum for this, if you have a recommendation for a better forum let me know.

Our kitchen cooking vents outside, and has an external cover similar to a dryer vent but it isn't sealing well. I didn't recognize how bad it was until the recent cold temps and now am trying to address it.

The vent tubing is 5", which is not a size they carry at Lowes/Home Depot. I did try to remove the old one to see if I could adjust it and discovered something alarming/concerning. When it was originally installed, the builder screwed the vent cover's tube INTO the vent that goes into the house. So, when I open the vent cover from the outside and look down the tubing, i see three screws protruding into the tubing from the outside and in an area that I can't get to without ripping the siding and potentially flooring apart because it's relatively deep (like 6") into the tubing. Why they would do that I have no idea, but what I do know is that I can't budge pulling the external vent cover off the tubing to replace it.

I'm contemplating my options and am considering things like using a Dremel or another similar tool to cut the portion of the screws I can get to and then drying to poke them out of the tubing, though the 5" diameter doesn't give me a lot of room to work with. I could potentially try to lift out the cabinets, but that seems pretty excessive.

Another option I was considering was to install some sort of magical flow regulator in the tubing that I have access to in the house, which would basically allow external flow when the fan was running, but would stop airflow coming in when the fan wasn't running. This feels like a good option all things considered, but I'm not sure if something like that exists or if there are guides for installing it.

I can post pictures if what I'm explaining isn't coming across well, and thanks in advance for your help!