The emerging technique of Mix-and-Inject Serial Crystallography (MISC) at X-ray free electron laser sources provides atomically detailed structural information about biomolecules as they function. Despite early successes, MISC is currently limited by the efficiency and robustness of the mixing injectors used to initiate the reaction and propel the sample into the X-ray beam for measurement. Here, we present a new method for fabricating the injector system that leads to simpler, faster, and more effective experiments. A mixing injector can now be produced from raw components in 100 min, only 5 min of which must be spent during the experiment, saving valuable time. The system is modular, enabling parts to be quickly exchanged in the event of unanticipated experimental difficulties, such as clogging. The injector holder is designed to be flexible, allowing each device to be optimized to maximize the number of diffraction patterns measured during each experiment. This holder has been used successfully during four beamtimes at two different X-ray free electron laser sources. Its robustness and ease of use is an important step toward making the MISC technique accessible and routine.