I like adding little expandable pockets to the back of my notebooks to put random stuff in there that otherwise would fall off. In the generator you can select the width and height of the folded pocket and the size of the accordion folds. Blue lines are scores.
A couple of examples:
This is a 3.5" x 5.5" field notes brand notebook and the pocket is 3" x 5" with 3/8" folds:
These are made with card stock but I think I’d be fun to try other materials like woven polyester or fabric to reinforce the accordion folds like in this other notebook I bought: