About ten years ago I bought these nut files that someone was selling on ebay, quite cheap. They were actually feeler gauges that someone had turned into a nut file by cutting some saw teeth (kind of) on them.
As I said they were quite cheap and not the fastest tool on earth, but they worked fine and actually I still use them! So for sure something like this would be good for you.
Unfortunately it was long ago so I don't have any records related to it.
There's a picture here:
https://ibb.co/3mLZv2t