Thank you everyone for the lovely comments!
I must know how you made the divider!! Will you pretty, pretty please, please tell me? I have been thinking about that for over a year but then I forgot about it completely until I saw this post. ^^;
I'll try to explain, but it was a lot of fudging.
Note: When I make bags, I leave a spot open around the top (rim?) of the bag to turn it, and it gets closed later when I topstitch. I've never tried the version where you leave a small hole in the lining and handstitch (?) it closed.
I made a panel a little shorter than the size of the largest rectangles minus the square cut corners, plus a generous seam allowance. The pink is the divider panel, and the black is the original large panels.
I made one piece of lining double the intended length, and put a piece of the outside fabric (tapestry type stuff) in the middle for stiffness and durability.
I then sewed the three side seams on the lining, being sure to sew it into all of them.
The biggest fudging comes when sewing the two diagonal seams. I did it by sewing half, being sure to push the divider away so it wouldn't get caught in the stiches where it shouldn't, and then doing the other half in the other direction, again pushing the divider away. I would suggest sewing both sides here and leaving the turning hole in the top. I can't imagine trying to handstitch it shut with the divider.
Later, when sewing the lining to the outside, it gets a little funky trying to turn the lining inside out, but it still works.
I hope that makes sense. Feel free to ask more questions, but my explaining skills have never been the best.