It looks like you sewed vertical rows last. (Just a guess) I would separate it into vertical rows and sew borders vertically between and on the edges. Not bias, that wouldn't help. I would use interface the borders or starch them for stiffness. Then soak it and block it. Since it is a hanger, you may get a few, waves. I would then sew a sleeve on the bottom and use another rod on the bottom to stabilize. I have used metal cafe curtain rods instead of wooden ones, the weight helps even out the waves. How do I know all this, I have been in the same situation. One of my favorite hangers have waves and with the bottom rod, they aren't as noticable because it hangs straight. Just my advice.