I have made several from other bags including an old handlebar bag and a 40 year old canvas army musette type bag. Since I wasn't smart enough to realize you can buy the frame separately I made my own, laminating 1/8 inch plywood to create something to slip over the bag mount on the frame. Other than their appearance and heaviness they work fine and although they don't lock on like the real thing they have never come off. I built these because the Brompton shopping bag I got with the bike is such an aero drag in a headwind. Sure does hold a lot although it doesn't work well with the S-bars unless loaded up a bit. Nowadays I usually go with a very light nylon mini-pack hung off the tabs on my leather saddles. No wind drag with that and mine is large enough to hold a tool roll, phone, wallet and a medium jacket. If I need something larger I throw on which ever of the front bags is the right size, say for 5-6 library books or the shopper for groceries.
Steve in Denmark is absolutely right in that once you figure one out it is easier and easier to make more. I can only sew by hand so I've been using fabric glue in lieu of stitching and it does not come apart. Plastic bottles can be cut into sheets that work well to stiffen otherwise floppy bags, BTW.