We report on the analysis and optimization of a binary phase element for shaping a Gaussian laser beam to a flat-top beam. Simulation results indicate that a single-zone binary phase plate can achieve excellent flat-top beam shaping quality similar to that achieved by using multiple zones. The degradation of flat-top beam shaping quality due to etching depth errors, deviation of illuminating wavelength from design value, and variation of input beam size can be compensated to some extent through on-axis adjustment of the flat-top beam observation plane. Experiments verify these theoretical expectations. The increased beam shaper fabrication tolerance can be greatly beneficial for low-cost prototyping and production of flat-top beam shapers.