Great bag! I like all the pockets! I saw a hint on another website recently about sewing "rubbery" fabrics and I thought you might like to give it a try. I haven't tried it yet, but will very soon. When sewing vinyl, leather, etc., put a strip of Scotch Magic REMOVABLE tape on the bottom of your sewing foot and I guess you could put it on the plate below the foot too. The sticky fabric will just glide over the tape without sticking. This tape is kind of hard to find and you have to buy a big roll. I found it at a Ben Franklin store and I "googled" it and Office Depot also carries it.
Great job! My grandsons need something like this for school. A suggestion for pockets: cargo or pleated pockets would give a little room for ID's, lip balm, etc., and would be easy to attach to the carrier. Put a flap over the pocket and secure with a snap, magnet closure, or velcro to make sure things won't slip out.