I saw an episode on tv (sewing with nancy - column quilts, I think). i think this method can be used to solve your problem. Cut 8inch strip of fabric for front, for back and 8inch strip of batting.
Check out this link. Use her sandwiching method to add 8inch strips to either side of the finished quilt. Then rebind it.
Be patient, it takes a bit to load.
http://video.wpt2.org/video/1701089053/