I have heard of the longarm folks sewing a piece around the outside to enlarge a backing such as you have and then removing it after the quilting. Maybe some of them will chime in on what they would really prefer. For simplicity's sake, I think I would add a strip lengthwise and one crosswise such that it makes an enhancement to the back and creates a new quilt for the back. I wouldn't put a lot of seams, just plain strips of either 1 or 2 fabrics.