Assuming you cut the fabric right alongside the ribs and the spar, then the easiest fix is to add some balsa to the edges of the ribs and spar so you will have some area to glue to cut out fabric flap back on to. Be careful, and try to get it as tight as possible. Sigment cement works pretty well to adhere the fabric flap.

When it is dry, use your heat gun to reshrink the fabric flap. Fill the gaps between the cut flap and the uncut fabric, spackle works OK. Then sand that carefully to smooth out the new seam. Now repaint as necessary. Sand between coats on the seams, and you can make them almost invisible.