Since the 80's, the Kahler Deluxe/Pro string lock has been the easiest and simple locking device! 

This genuine Kahler Deluxe/Pro levered string lock, which mounts after the nut with 4 screws, can be used interchangeably with the 5513 standard string lock and shares the same hole pattern. Built to last, the design has been revamped to be tougher and last longer than the previous models from the 80's.

It measures 1-5/8" wide and will fit on almost any headstock.