Today I steeled myself, and continued on the codex bag:
And I have also found something very, very useful in the vast depths of my workshop:
I don't know, what they are originally for, I assume, for making bags. They are hollow hemispheres with two holes on the sides (I strung rings through them) that have "feet" you can fold to the sides after sticking it through the leather, and they have matching plates they can be reinforced with.
Now I only need to sew in the zipper, and make the lining.