Recently I ran into the same problem with the backing not being big enough to allow for the eight inches the long arm quilter needed. The backing was big enough for the quilt top with about two inches to spare. I cut a wide strip of muslin and sewed it to the backing. This gave the quilter sufficient materiel to put the muslin on the rollers and it worked out perfectly. When I got the quilt back from the quilter, I trimmed the quilt and the muslin never showed. It just gave the quilter the extra fabric she needed for the rollers. If you don't mind which way the grain of the fabric on the runs (up and down, or crosswise), this might work for you. Just a thought.