Replacing Battery Area

The part I am talking about is the vent opening that comes down from the passenger side of the front windshield. If you lay in the passenger floor board with your head toward the engine compartment and look up, you will see the opening. Every parts car I have has rust holes in this area, and If yours has a goodun', please let me know before you cut it loose(I will send a picture showing hw much I need). I want a correct repair even where it will never be seen by anyone else.

Will