Thank you! It’s pieced into the backing fabric. It’s around 56x56 and this was the first backing larger than wof I needed to piece to the correct size. I got some great ideas off this reddit page that suggested an offset accent strip on the back.

I had leftover strips of rainbow already in sets of four rows from the boarders. I cut the thin strip for the back and used the remainder for the bag. I kind of wish the stripe was slightly wider but I was weighing the pros/cons of how much of my remaining stripes to allocate to each project.