“a snap in vent to fit back in after working”?
What happened to repairing the ceiling so that it cosmetically matches the original ceiling?
You cut a hole and put in a “snap in vent” (a/c return air grille?) and leave the ceiling open to drafts and air movements throughout the second floor system. This affects the fire blocking and draft stopping of the structure. If you are not careful the entire ceiling could become a return air plenum and then then duct work, plumbing pipes, electrical wiring, the structure, everything, would have to be plenum rated.
Scary thought on this end.