You could do it the same way a facing is done on a garment neck or sleeve. That is, lay out your top upside down on the right side of the backing (so "right sides together"), add batting as you like, stitch the seam along the edge of 3 sides and most of the 4th side, then trim the seam allowances on the backing/batting. You can "birth" it by turning right side out. You'd need to finish the last bit of the 4th side by hand.