Bone nuts.

[fender3x]

The nozzle cleaner files have been mentioned by others as OK for cutting bass nut slots, but the sets I’ve seen don’t have the diameters for cutting slots for the E and A strings as they only go up to 1.5mm diameter (which at 0.059” is slightly too small for a 0.065” slot.

I made a mess out of a blank with needle files. That may just be me. If I had better hand-eye coordination I would have learned the violin. (my mother thought I'd never have the dexterity for it...so...bass). I have found the nozzle cleaners to require a lot of patience, but with that proviso they have been darned near idiot proof on my bass nuts, at least for this idiot. BUT, you do have to search for the right set. Most are like Simon says. The biggest one is not very big. You can find bigger ones. I was lucky enough to find a set that was specifically for 5 string bass, so it had sizes that worked well for E, A strings. There's one for the low B string too, but I have never used it, not having a 5 string bass...