If you dont mind using different sizes you could make the middle block a 3 unfinished square and the corner blocks 2.5 square and the four side pieces 2.5 by 3. When you sew them all together into a 9-patch, its 7 square. When you cut down the middle both ways and sew it back together you lose .5 each way so you now have a 6.5 unfinished block.