Whether you use a border or not should be decided by the finished design you desire on your quilt not on the width of a piece of backing fabric. A piece of fabric 54 inches seems like something other than quilting cotton-which can be fine. However, you can certainly add to the back by cutting vertically or horizontally at some pleasing dimension and inserting a strip of another color, something you used on the quilt surface or unused blocks, etc. Just don't let the dimension of the back be the dictator or your quilt.