We are almost done with our backpack. Sewing the back layers together can be cumbersome, especially when using thick padding. I recommend using some kind of adhesive to hold all the layers together while you top-stitch the back piece.
The top-stitching design I included in the pattern is nice, but you can use many other designs. I have sketched out a few. I also tried out a few of them in my padding post.
I used the original design on my two pink backpacks.
For my blue backpack, I tried something different.
The adhesive I used did not like the high-density foam, so I had difficulty keeping the layers together. I love to see what designs you try for your topstitching.