I have a couple of tubs of fabric that I used for cushion linings. Some of it is reclaimed and it's perfect for making things that might go wrong. It's much better to make a mistake with fabric that cost pennies rather than pounds.
Although neither of these versions are lined, it was enough to do a video so that I could explain the features to Katie and get her go ahead to make the final version. In case you missed it before, here is the video of the completed bag.