I came across a rectangular sheet of relatively rigid plastic. It was just the right size to form the bottom of a bag so I used it, just as it was to reinforce the bottom of this bag. I sewed it in between two layers of black straps so the bottom is not only rectangular, which is unusual for my bags, but is also stiffer so it stands up straight.
Also, this is the first bag I have sewn since the county changed recently to black and orange rather than yellow-green straps. Since I can only use what they bring me, I now have lots of black to inspire me.