However...mistakes were made...had to take out stitching and rethink and re-cut on several occasions.
Here is the stuff I use for the rigidity of the box:
I created hinges with an extra piece of fabric to connect the bottom with the sides. I didn't have a large enough piece of the pelon o cut the bottom & sides as one piece. I rather like the hinge idea anyway.
Then I discovered my side pieces weren't quite big enough. Happy accident as I added pieces from a different fabric to make up the difference and like the result. All the exterior fabric is left over from upholstery projects.
Here is the finished product.