I used screws and washers with the cloth. I'd rather have it as secure as possible before they're in the coop than to learn the hard way. I can see if something happened while freeranging but not for something to get into the coop.
Just finished covering the larger ventilation openings with 1/4 hardwire cloth. Above them, around the beams, are smaller gaps. About 1-2" high and almost 3" wide from the wall to beam.
From outside:
Inside: