About ten years ago I was in Gyimes (Transylvania) and watched a Moldavian Csángó koboz player customize a student's instrument (probably from the
Hora factory in Reghin) for the local style. It just took a pocket knife: he shaved the nut down to remove the grooves, then cut new ones into the
bridge.
I think the player was Szabolcs Csibi - someone who knew what he was doing.