I use a strip of sandpaper folded over so there is grit on both sides and clamp it in a vice. That is about as narrow a file as you can get. Then I mark the back of the ring with a felt pen 180 degrees away from the gap and align it with the vice jaws so, as I am drawing the ring towards myself I can keep it straight, no wiggle while I am filing.
I found the ring gapping tool mentioned above creates a gap that is not square when the ring is compressed.