IMHO, almost all of my backings have had to be pieced one way or another on my quilts. In the 70s when I first learned to quilt, they told you to cut your backing into 3 pieces instead of 2 and then center the seam lines on the back. If you have more of the backing fabric, I would just cut the length in half, put another piece of backing in between and then cut the back the size you want. 2 seams and you are done. Easy, peezy!