In my mind your problem comes from stitching straight-grain fabric to bias edges. I have had that problem too. While I am not a very experienced quilter, I am an advanced sewer. I would measure the center panel in several areas, take that average, and then cut the strips to that length, then pin, pin, pin. I find if you pin the ends to meet and then work from the center edges out to the ends, this helps. Also steaming before sewing helps to shrink problem areas.