Glad the pattern is working for you. You can do a row of leftover pieced blocks down the backing to make it wider. Don’t try to center the row but off set it to one side. Trying to keep a row centered on a backing is tough.
I use a zig zag in off white thread and butt 2 pieces of batting together and sew down the center. You may need to straighten the batting edges before you start so they go together well.