You can try stitching over some of the seams tucking in just a thread or two more along those edges. Remember that even though the adjustments seem tiny, each one you do is double because it is two thicknesses of fabric. Think I'd try these "narrow mini darts" about 5 blocks deep, maybe every 3-4 rows. Probably only need a couple (3-ish is my guess) each side.
When I do projects like this, I also find it easier to graph it out and make block units but I don't do hand work.