This photo shows how retaining screws can be used to adjust the "break angle" of the strings - the angle at which they cross over the nut/zero fret. While this photo shows an unfretted cigar box guitar that uses a threaded rod nut, the same concept applies to a guitar using a zero fret.
Guitar Fair, 11-14 September - Costa del Sol Searching for the best luthier of guitars & basses... G-Ray Guitars / The Cigar Box Guitar Shop - UK The Cigar Box Guitar Shop is the only store to stock G-RAY guitars. Each one a hand crafted original, with a unique look and sound. http://www.cigarboxguitarshop.com/