The threaded inserts will work fine, but it's overkill. It's not like a cowling has flight loads on it. The inserts are strong enough to hold wings on (assuming the wood they are threaded into is strong enough) so putting them on a cowl is simply more effort than is required. A hardwood dowel glued into the balsa with the screw hole drilled into it and then the threads hardened with CA is quicker and cheaper and still strong enough to never get loose or pull out.