We suggest that properly designed masks altering the rotation-integrated flux arriving at the substrate may be used to improve thickness uniformity in a variety of crystal growth techniques. As an illustration, we calculate the dimensions of the mask required to produce a perfectly uniform flux for a single group-III species in single- and multiple-wafer III-V molecular beam epitaxy. Estimates of the detrimental role of errors in mask machining and position are provided and schemes for correcting flux nonuniformities for multiple sources are suggested. Our results imply that flux masking can lead to significant uniformity improvements for growth on single or multiple wafers.