Use a pocket protector. I bet those are fairly universally sized.
Except no one probably knows what they are anymore except for hardcore nerds.
Here in Sweden, the most common use today is by nurses. Of which there are more than the particular pocket-protector-worshipping class of nerd.
/has fountain pens but no protector..
Ah, the fountain pen as an everyday tool has gone the way of the Wooley Mammoth. I was in a large main police station (John St. nick, Brighton) in the late 80s toiling over some computers in the incident room and a visiting dignitary (the late, unlamented, Margaret Hilda Thatcher) wanted to sign a commemorative letter. A constable was dispatched to run around the whole station like a frightened rabbit to find
someone with a fountain pen. Eventually he found the only person in the building with one, me.