The Legume Notebook holder is my favorite bag I've ever made. It's an incredibly simple bag that solved two problems in my life, destroying notebooks and loosing every writing utensil I've ever known... Maybe it could help you too? Designed to hold an A5 notebook (5.75"x8.25") along with a grip of pens, pencils, markers, paint brushes, etc. Simple 2 step fold and elastic closure ensures no loose pens or notebook explosions while in between here and there.
