Really like your backpack. Nice job!
I'd be careful using it as a " back" pack at a quilt show, wallets have been known to be liberated from them in crowds. You can wear it on your chest to keep an eye on it, still freeing up your hands.
Re the stiffening -you can use peltex or that really stiff interfacing type of product. It may be fusible, but you don't have to fuse it. Alternatively you can use those inexpensive flexible plastic cutting boards from the kitchen section of the dollar store. You may have to make an extra layer of fabric to slip it into to hold it. If you use them, cut them a little smaller than the area you want to stiffen, then it will be out of the way of your sewing machine needle.