Make your backing bigger. I almost never have a plain backing. I use it as a place to use the bits and bobs left over. For that small of a top, I might add a diagonal stripe about 6 or 8 inches wide. This would allow for plenty of room to trim it and not have seams to close to the corners. You could use a complimentary piece of fabric or sew together your leftovers to make the strip. Another suggestion is to add a 'cross' in the center - 4" band in the middle across and 4" middle top to bottom.