I, too, piece my backings. Have lots of fabric that I need to use up......time is running out.
Since most of my quilt backs need to be more than 45 in wide, I usually cut the 45 inches in half lengthwise, and add different blocks of fabric leftover from my quilt in the center (or even some orphan blocks that look OK with it) then sew the two "split" pieces onto the center blocks, etc, giving it the width that I need. Takes a little measuring of course, but then all quilts take some measuring and math.
I have also seen pieced quilt backings that were not even in the same ballpark with colors, etc as the quilt top itself, and they looked wonderful, too. Takes some imagination. Sometimes I have it, and sometimes, I don't.