If you make a neck with a zero fret or no frets at all, the optimal nut is just slots in the fingerboard.
There is no abrasion to bother with if the string is linked off by a zero fret, hence no need for a loose/replaceable nut.
Fretless sounds best with a wood nut, which is flush to the board. However, there may be some abrasion form the strings on softer material. Hence, the best solution is to cut slots and harden the surfaces somehow, e.g. CA or epoxi, which are easily renewable.
IME and IMO, of course, but it really is MHO!
