You have a couple of options. I don’t know what type fabric you are using so you will have to adjust. You can sew fabric and make a seam in the back, but note you will be using a seam allowance on both sides that wasn’t there in orig pattern. Make a 1/4 seam (1/2 x 2 = 1/2″) used up. You will have to reduce the seam allowance on both side seams to make up this usage. If you have a serger, use this to make seam and it will avoid raveling. You could also use a strip of fabric and sew over the seam like an applique.