You could also use that..........
clear packing tape, then you wouldn't notice it as much when you're on stage...... I knew a guy once who played a jazz box mostly and had a couple of homemade "feedback busters". What he did was cut some foam that was about 3/4" thick to the exact size as the F-holes. He'd traced the shape of them before hand on paper, cut 'em out, and used a sharpie to draw the shape onto the foam. Then he'd cut out two slightly larger shapes on some plastic, glued the foam to it so just a bit of the shape of the plastic was out over the foam. The plastic was already black so he took some ivory colored paint and painted around the edge of the plastic so it'd look like binding in the F-holes. If you'd made them like this, rhen you'd be able to take them in and out at will and they'd do the job. You could also do this with a couple of pieces of clear plexiglass if you wanted to.........JH in Va.
Ralph Mooney rules!!