Hi David,
The divider only goes up roughly half way the height of the bag. The welt up the sides are made from the same thickness material. That way when I cut it all flush, sand the edge smooth, bevel the corners, dye it, polish the edge and wax, you don't see the divider or the welt. It just looks like I used 50% thicker leather than actual. I did sew down the bottom of the divider to keep it from moving around, but you don't have to. Before sewing it looks something like this.....
Sorry for the poor drawing. I think you can get the idea though.
My dad told me this about leather, "The final 10% of the work takes 50% of the time and is 90% of the look." That's what it feels like. If you miss architecture, stitching, dye/finishing or edge finishing, the overall look suffers. I am no expert at crafting things, but I have been around nice leather all my life and know what good looks like.
God Bless, Marc