I once replaced the nut on one of my classical guitars by superglueing two pieces of bone saddle material together to get close to the correct thickness and then slotting it with a set of tip "cleaners" for an acetylene cutting torch, which are literally about 2" long rod shaped files of various gauges. B/C the cleaning files are so small, it was slow going, but that was OK b/c I would fit the nut in place, holding it down with the strings, in order to check the progress as I went.

It worked for me, but I'm sure the fancy, schmancy correct files would do a better job and more quickly.

As it is, the guitar is still in play and happens to have the best tone of any of my classical guitars. I don't necessarily credit the nut; it was just the only option I had in this little, podunk TX town where nobody had ever heard of a bone nut, but they had some bone saddle blanks in a drawer that hadn't been opened for 20 years, somewhere in the back room.

Dugly