TL;DR: Faux Midori Traveler's notebook↑ (fauxdori) is easier than people make it seem.
I've had this heavy tooled leather refillable address book from Oberon Design↑ for many years. The address book insert is long gone, and I've attempted to use it as a refillable notebook over the years, without much success, because the original insert was a non-standard size (A5, but a bit more narrow).
Regular A5 notebooks would not quite go in all the way, and would buckle if forced, with the spine ending left of center on the cover, so that it couldn't be closed completely. I tried using smaller sizes (like B6), but they just slid around inside.
One A5 insert will not allow the cover to close completely. Two makes even this impossible.
I bought a generic traveler's style notebook a couple of years ago in B6 size. It has been lovely, but I wanted to carry a slightly larger notebook for a while, and remembered my lovely old Oberon cover.
Click the image below to see the full collage.
This time, familiar with the elastic system for holding inserts, I added elastic cord to my old leather cover, and presto! Just enough slack for an A5 to be properly centered. Since the insert is centered, the cover can fully close.
Click the image below to see the full collage.
I used one long strand of 2mm elastic cord for this. There's a square knot on the spine, which lays very flat compared to the metal beads often added to clasp the elastic on traveler's notebooks. I used a scrap of leather from a sewing project to make a tab to protect the edge when closed. Very simple, and will hold a couple of a5 inserts nicely. Also pretty cheap and easy to replace the elastic if it wears out.
I did toy with adding other adornments, like knotwork buttons and beads, but I prefer to keep it lying flat when open. You could easily use elastic that was a bit thicker, but I wouldn't recommend anything thinner.
