I have carried Moleskine notebooks for may years now and have found myself favoring the small, cardstock covered, pocket sized cahier notebooks for portability but after a while living in my pocket, they get a bit tattered and torn. A first solution to this would be to just use them up faster, the second solution was to design a cover that would protect them. The first iteration that worked functions as a slipcase, only adds a few millimeters to each dimension of the notebook and seems functional. I added a grid to the front and back of the cover to add visual interest.
Source code SCAD file is included and optimized for Moleskine Cahier pocket journals (6/23/2017 : Field Notes journals also fit perfectly). No reason it can't be adjusted (if needed) for similar sized notebooks. I'll probably update this into a Customizer once I clean up the code and pull over what is needed from my standard library.
--- Update 06/21/2017 ---
Now a Customizable slipcase.
I used 50% infill on my first print but with 2 perimeters, there's not much room for infill so choose infill as you like. I did toss a pair of 20mm helper disks down at each end of the slipcase because, printed in it's exported orientation, it is long and thin on the build plate and made up entirely of 90 degree angles. The helper disks minimized the shrinkage on the ends such that it's only noticeable if you know what you're looking for.
Since I added helper ears to the print, I carved them off with a scalpel and finished up with a sanding stick.
It took a considerable amount of tweaking to get the parameters just right. The Moleskine cahier notebook I designed this for fits quite tightly. I printed several 1mm test slices of the design to ensure the opening was wide enough. It took four or five times to get it right. If you adjust the sizing, I recommend you do the same.