Mock-Up

There was never a worry about how the divider should look in the bag. My concern was getting it sewn in at the bottom and the sides. I wasn't sure how it would go together. So I decided to do a mock-up.
The key was in how I boxed the bottom of the bag. There are two ways to do it.

One is to line up the side seam with the bottom seam, and measure out from the corner, draw a line perpendicular to the side seam, and stitch. The other is to to cut out a "notch" in the corner, then line up the seams and sew.They both look the same in the end, but cutting the notch helped me to figure out how to get the divider panel attached to the sides and bottom seam of the bag.It turned out to be a classic "sew into the corners" kind of a problem. I had to "box" the bottom first, then sew the bottom seam, catching the bottom edge of the divider between the front and back panels.

Now that I have that worked out, finishing the bag will be comparatively simple.