Diffuser heads whose angular response is proportional to the cosine of the zenith angle are needed for global irradiance measurements. Various material samples were measured for their transmission properties to find out the most promising material candidate for use in an improved solar UV diffuser. Novel bubbled quartz materials were found to be attractive alternatives to the traditional PTFE materials for this purpose. A 3D Monte Carlo particle tracing software was constructed and used to optimize the diffuser design. Integrated cosine error of 2.3% was measured with a raised flat diffuser on a prototype detector. The preliminary simulation results indicate that integrated cosine error can be lowered to 1.7 % or below simply by adjusting the dimensions of a flat diffuser. Integrated cosine error of 0.8 % or below can be reached with shaped diffusers.