Andy comments:

Another trick that I have used is to buy a $3-4 air condx vent
cover at
Home Depot and put it over the access hole in the sheet rock. That
way
you can go back and do stuff if you ever need to... When painted, it
looks like just another air condx duct in the walls....

Probly isn't a good idea in your case, but I thought I'd present
the idea.

Andy in Eureka, Texas