I found a formula that tells me what size I need to cut to get the finished product.
I used a fusible fleece to help hold its shape but it's still pretty floppy. I hoped to have enough material left for a handle but didn't. But I'm going to take it apart to put one in so I can carry it with ease.
This is what it looked like after cutting out the corners. I was convinced I'd messed it up, because I'd never cut such a large corner before, but it came out exactly the size I wanted it to be.
I got to try it out today as I rode my bicycle to knitting group. It fit great inside my reusable reversible shopping bag.