I love that pattern and your colors, too.
If I were making it, I would cut all the outside rectangles at 8 1/2 inches, and sew them on with a partial seam on the first addition. I made a "square" quilt with about 350 squares or more, and they are all partial seamed. I loved doing it and loved how it turned out. I know a lot of people are afraid of partial seams, but I really enjoyed doing them.