Maybe I’m missing something, but wouldn’t the Jura Tool sanding film that Julie mentioned in this previous thread solve this problem? You can get it in a variety of grits and can cut it down to any width. I use a similar product that’s used in the dental industry.
One other thing that I’ve done in a similar predicament is that I’ve taken a fine needle file that is the right shape, but is too big to fit into where I need it and ground down the outside to get it to fit. Since most needle files have two sides, it’s kind of sacrificing a needle file and making it be a one-sided file, but that’s sometimes worth it.
Hope that helps!!
Jeff