I saw an interesting idea the other day where someone bought a set of feeler gauges and then apparently filed teeth into them to create a cheap set of nut files. Most gauges probably won't have blades thick enough for strings beyond ~0.040, and they certainly wouldn't be very robust (if even useable)... but, it's an interesting idea and it's possible it could work for those wanting to do a one-off custom nut without lavishing money on proper nut files.