Hi again, if you use the fret style spacer I would recommend [as mentioned by Wayfarer] a larger fret so you can cut the slots deeper and help with the break angle over the Zero fret and the pressure down on it, although it will not be much. The angle the strings take from the spacer fret nut to the tuner posts my indicate how deep the slots may need to be or if a spacer nut is needed. You can always use string trees to get a downward angle behind the nut. Cheers Taff
Hi, Just one last thing that I consider and it is the height of the strings over the first fret. For my slide guitars and my style of playing, I prefer a slightly higher action there. It could depend on the stiffness of the strings, playing style and weight of the slide as to how high/low I set first fret action, so as to eliminate too much string to fret noise/rattle.
I remember on my early acoustic guitars back in the day, making bone nuts with a bone zero fret incorporated in it. Too much work but a good talking point.