Ty,
I would lay the corners flat, spritz with water. This will allow the fabric to shrink slightly. Encourage the stretched area back into the correct shape.
If this doesn't work to your satisfaction. Shorten the stitch on your machine and stay-stitch within your binding allowance. This should also causing shrinking.
OR you could use a longer stitch and then draw it up like you would when gathering an edge. Then distribute the fullness within the gathers.
Recreate your problem with like fabric and shape, and test, test and test several of the suggestions and see which one will work best before working on your "real" top. Good luck. AND, please let us know which worked for you so we can all put same in our memory bank. Suzanne