Reddit reviews Midori Traveler's Notebook Journal Passport Size - Brown

You might try a Midori Traveler's Notebook or google any of the cheaper Fauxdori options from various vendors.
This pen loop will fit a pen inside the loop and this pen loop will fit the pen's clip in the loop.

There's also a Passport size that's about half as tall.

Also, I'll second the Leuchtturm1917 it's a great notebook, and the paper is more fountain pen friendly than Moleskine (if you're in to that sort of thing.) Although, the Moleskine sketchbooks come with some nice thick paper.

From what research I've done, this may be the solution to your particular problem:
https://www.amazon.com/Midori-Travelers-Notebook-Journal-Passport/dp/B003SNV0YQ

Here's a video of it being used so you can see what I mean:
https://youtu.be/JEds4O-RLWg?t=513

Occasionally I do some research about minimalistic leather journals because I always seem to have a reason to write things down but dislike the common 5-star notebooks and the like. There are lots of beautiful leather products out there especially with many upstarts through kickstarter and Etsy but it seems like the one everyone is trying to replicate is the good ol' Midori. Simple and beautiful and with people figuring out ways to cheaply get refills for the paper, I think some of you may enjoy it.

Also, the smaller "passport" size: http://www.amazon.com/Midori-Travelers-Notebook-Journal-Passport/dp/B003SNV0YQ