If the fabric was less than the desired size, then I would put a narrow strip down the middle and perhaps across making two large blocks or a border around it. Folks will call you creative.....To me, fabric has never been consistently the same width even in the same fabric line. I have been to a fabric manufacturer and know the greig goods comes from different surces and will shrink differently in the printing/dying process. Some will say "live with it and work with what is there" I have been told by fabric companies they only guarangee width within a certain range. Also, does the receiver of the quilt demand they be an exact size - say 43"? Also hen I have backing that is slightly less in with than I need, I will just sew a strip of contrasting fabric down the middle to make it wide enough rather than by 2x the width and have the extra sitting around.