I'm really not sure what your fear is. You play cello and have played NS. I think it is the fear of playing without lines that holds us back. I've been there. All you really need are marks at 3rd, 5th, 7th, 9th and octave. That devides the board into smaller areas which might make it easier for your brain to work with in the early days. So as has been suggested side dot markers or even small lines that just hook over the edge of the neck would do.