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? This "lite" version is built from various ultralight/durable fabrics and sewn to hold a "pocket" sized notebooks. (3.625"x5.875") 

This one is for the thru-hikers, back/bikepackers, commuters, and the ultralight freaks out there... It's a great solution for keeping your notebook and utensils situated while you're out for a long haul.

6.25" x 4.25" when shut.

Sewn with love in Oakland, California.