You don't cover both sides. What airplane is it on? I wouldn't waste the time doing it on most. On my gliders, I do monokote hinges on the flaps and ailerons to close the gaps. I have seen people use clear plastic, and glue it to the trailing edge and lap it just over the control surface to cover the gap. Keep close tollerances on your surface and you don't need a gap seal. Just my 2 cents