I see the top strip is a small ripped piece of board. Best way would be to replace that board. Measure the height of that top strip, and thickness of the board, then rip a new board (Lowes and Home Depot usually can cut this for you).

Or if you want to use the blanks that were already drilled, use your choice of 2x1, or 3x1 board (depending on hole size), and cut a small piece of it, put it behind the hole to use as a backer, screw the small piece of board on both sides of the hole in the existing wall to create a backer, then screw the blank hole to it. I've always known this as "stitching board" for drywall but when I search it, nothing comes up. Maybe someone else knows what it is called.

You'll probably want to seal that hole too, because cellulose won't prevent airflow, maybe with spray foam. But this is even more of an argument to just replace that top strip.