Personally my vote is with the suggestions of adding extra into the center area of the backing. Be creative with fabric and placement. Use 1/2” seams pressed open then if they happen to align with seam in the top there is less bulk. No worrying, gluing or stressing on how the needed extra will align with edges of quilt. I use this principle when clients have pieced extra strip on the backing. I start with the narrower added strip. That way I never end up with the seam only a couple of inches from the end of the quilt. IMHO it is always better to have the seam further from the edge.