That is a pretty pattern, but hard to add to. I like what Gay came up with. This is a couple of alternatives.
If I were determined, I would add one more block to the borders on each side. If the blocks in the center are 12 inch, that will give you an extra 4 inches to fill. To me your fill has two options. You can add your thinner border of 1 or 2 inches and then fill with your background fabric next to the border strip, thereby floating the outside border and then add the same size float of the background to the outside of the border.
Or, you can add background fabric on either side of the darker inner border and float the darker strip. Then however wide a piece you added right next to the border on the inside, should be added to the outside. Or you can add the same type strip to the outside of the border which would create another floating lt background, dark, light background.
I look forward to seeing what you come up with.