Rigby and Pellar ..(get me!) pinned a back band for me once to get smalled at home (day tripper) it was pinned at the underarms, nearly at the cups but still on backband iyswim. My seamstress auntie sewed the folds for me.
So, get a mate to fold and pin, each side equal. Mind out for pin stabs, natch. Then stitch down each fold firmly. Job done.
You can get the rixie clip or it is easy to take down one size by removing the clips at the back, cutting a very small amount of fabric and reattaching. Any more than one back size tends to distort the shape too much though.