If you have enough scraps from your rows, you could cut them into piano keys or squares and add borders of same to just the two sides to make it wider. Because your quilt is busy, I would use a tone on tone or blender fabric to encase the squares or piano keys. That would give you 3 rows to your border on each side so you should be able to add all the width you want working with that.
For example, if you cut 4.5" squares or piano keys, and 2.5'' borders for them, you would be adding 8" to each side for a total of 16".
If you don't have enough scraps, you could pick 2 blender or other fabrics that would go with your RxR and make the triple border for it with 9P cornerstones to add interest but not so much busyness as to compete with your main quilt.