It's considered blasphemy in the classical world, but I add side markers to all my nylon string guitars at 5, 7, 9, 12 & 15
You can get rub on marker at String by Mail. They're great for trying out where you want to add them before making a permanent decision
They sell them in several different styles.
https://www.stringsbymail.com/rosett...dots-7878.html