I made some mods, and there are also a couple things I might do differently next time, so here they are:
Interfaced the cotton lawn fabric pieces twice. This worked perfectly to build the lawn up to a weight and firmness close to the denim. I applied one layer of Pellon SF101, let it cool, and then added another right on top of the first layer.
Applied the fleece to muslin and quilted the fleece/muslin to the lining. Applying the fleece to muslin is a great way to add another layer for structure. If you don't have muslin use some of those prints you are feeling over or wonder what you were thinking in the first place. Do make sure any color/print won't show through your other fabrics.
For the quilting, just a few lines is good. I start in the middle and then quilt to the side about every 2". The Box Bottom Gauges are useful for marking out quilting lines, or eyeballing where to sew them works too. No one is going to notice if they aren't evenly spaced, or even very straight! But it is good to stitch all these layers together by quilting them and makes it easier to work with these pieces if everything is held together.
Increased the box bottom corners by using the 1.5" gauge. Super simple, but I find I have to remind myself to mark out the new corners or I end up having to cut them a second time!
Used the zipper technique from the Improv Zip Wristlet pattern. This worked out well, but next time I'd add a 1/4" to the total zipper length. These pouches have slanted sides, which means where the zipper is attached is actually slightly longer than the top edge of the pattern piece. The zipper does open the width of the top of the pouch, but it could use just another 1/4" to totally clear the top edge.
Finally, for next time I'd probably topstitch the lining side only. Or maybe topstitch around the entire top if I wasn't using this bright pink YLI topstitching thread. I'm not that excited about the way the topstitching around the top looks for this particular bag. It's not bad, but it distracts me because this pouch is visually oriented vertically and the horizontal line of the topstitching next to the zipper catches my eye. Picky, picky...!