A single pen loop is common, at least for Personal size Filofaxes. Often, they are a little ungenerous, refusing to accommodate wider pens like the Frixion. The solution is a Leuchtturm 1917 pen loop. Here it is in its packaging: I attached mine to a transparent envelope, near the back of my Filofax and stuck it so it protruded a little: Note how it accommodates the wider Frixion without difficulty. The slight protrusion is important so that your pen clears the divider tabs, like this: And here is how it looks with the binder closed: And finally, from above: The Leuchtturm 1917 pen loop costs just £2.50 or so and I am finding it makes a big difference to the usability of my Filofax. I got mine from Cult Pens (no affiliation, just a happy customer.)
