you could 'self-bind'. Have the batting a shy 1/2" bigger than the front and the backing 1 1/4" bigger than the front. Then you fold the back over the front and machine stitch it down. I like to use a decorative stitch since it shows on the back and front. Being just the backing, it is easier to fold and make your sharp corners, whether you mitre or just fold over. I often use this same technique when making baby quilts because then they can never chew off the binding!