Leaky around the fresh air vent

[Predist]

Hey guys I have found the most horrible rusty are of my comet it is around the fresh air vent. It is on the left side under the fire wall and the dash.
It is so rusted out that it was one scew holding the entire vent in place, it has rusted out all around the vent, now I have one big hole there with
no idea how to close it off, let alone how to get clear access to it to repair it. Any ideas at this point would be a plus. Thanks