@terri Thank you! In case you do decide to go for it, here's my working sketch of the measurements for mine, maybe that helps a bit with getting started 

@terri Thank you! In case you do decide to go for it, here's my working sketch of the measurements for mine, maybe that helps a bit with getting started 

@milla Yes! It's absolutely possible to write something down while just holding it with one hand without any additional support when folding the wallet part to the back (very nice for quick notes on the run), which is definitely helped by the cards.

@ann3nova The paper inserts themselves are Traveller's Notebook refills in Regular Size, which is 110mm x 210mm. That's the same height as A4 paper is wide, and about the size of A4 folded into thirds.
These are the dotted ones in white (026), but they're available in many different paper types.
Traveller's company offer leather covers and various accessories, there are also compatible covers and refills from other brands.
My own cover has a total opened size of 390mm x 220mm. More details once I get around to documenting it properly, but for now here's my sketch with the measurements in mm 

@schnittchen thank you!
By the way: a few days ago I finally finished my custom traveller's notebook cover / wallet!
All in all I'm very happy with how it turned out. There are a few things I might adjust or do differently for a V2. For example, the money compartment could be a little wider and attaching bias tape straight is harder than I thought.
I'll have to tinker with the closing mechanism: in theory, there are magnets and ferrofoil in there, but although I tested it and thought it should work, in practice the adhesion through the two layers of leather is so weak that you barely notice it
Maybe I'll try bigger magnets or a magnetic button.
But still, most things work just as planned in real usage and the aesthetics work as I hoped! I enjoyed working with this cork leather a lot (sadly the shop I got it from closed down, so I'll have to look somewhere else for fuzure projects).
The hardest thing for me about this project was serial decision paralysis. Since I planned this all by myself from scratch, every single measurement and design was a decision I had to figure out. Which was especially hard to do starting from a blank page. For my next project I'll probably do something guided with a pattern to recuperate 
If and when I'll find the time, I'll sort out and upload my plans, in case anyone wants to make something similar themselves 