Basically the Immersive Engineering cloche, but for mobs. Visually, the mob would start small and grow within the tank. When it reaches full growth, it would start producing drops. There would be some maintenance and risk involved. The machine would require RF, and the mob would have to be fed with nutrients and sedative. If the mob ran out of nutrients, it would not produce drops and eventually die. If the mob woke up, it would break out of the tank and terrorize your face!