Have you thought about 4 strips of elastic? Casing over each shoulder and across the front and back bodice. Your casing would be mitered where it meets at the shoulders or not. Whichever is easiest for you. Insert the elastic, sew the ends down and viola. Square neckline.

Thank you. I'd thought of four pieces of elastic, but not of sewing them together and then inserting them in a casing. I couldn't get past the idea of them anchored at each end with the thread showing then on the outside. No wonder there weren't any instructions for elastic. Dead easy unless brain dead. Thanks again.

Can you permanently gather the front and back sections and just put elastic in the shoulder sections?

Four pieces anchored at each end is exactly what needs to be done. There is no reason to have four pieces if you're not going to do that and no reason to cut up the elastic and sew it back together again. You can blind stitch the ends down so that it doesn't show. At any rate, you'll have to sew down the casing openings where you insert the elastic which is going to mean some thread possibly showing. OR,

You could make your casings before putting the blouse together, insert the elastic (4 pieces) anchoring each end of the elastic in the seam allowance. Sew your blouse pieces together and face the backside with either matching fabric or bias tape to cover any raw edges at the neckline.