A simple model is used to design and optimize masks that when etched onto the aperture of a normally multi-mode gain guided vertical-cavity surface-emitting laser, VCSEL, cause the device to lase in a single transverse mode. After etching, Gaussian beam profiles for the LP01 mode are achieved over the entire operating current range with greater than 16 dB modal suppression. To date single mode emission at 2 mW has been achieved with pinning of the nearfield width. The modelling shows that the etching introduces a spatial filter on the higher order modes. The power in the fundamental mode remains constant before and after etching.