You could add a row of the smaller squares on point with the same color cornerstones. I would do this to the top and bottom. Then a small border (the sash size) of a color (maybe a different one than the border squares if you only have the 2 fabrics) followed by a white border of same size. This should make it the size you wish without it looking like an add-on. Then you can finish with whatever border you want. Please post of pic of what you decide to do.