Im thinking make a divider the size of the bag, plus seam allowance all around. Use batting and interfacing to stiffen to your liking. Sew the three layers together at one seam that will be the top. Turn, press and topstitch, if want. Quilt pc to your liking. I would sew this into the side and bottom seams so It divides the bag in half. If you want a boxed bottom, youd have to play with the placement so the sides arent crunched. I hope this helps, its hard to put in words what I see in my head. Lol