Smart Instruments are a novel family of musical instruments that embed sensors, actuators, wireless connectivity, and semantic audio technologies. This paper reports the findings of a participatory design approach to develop a Smart Cajón, a box-shaped percussion instrument with Internet of Musical Things components. Five initial co-design sessions were conducted with different professional cajón player participants. The players were invited to devise tangible mock-ups by placing provided sensors on an acoustic cajón, and to express desirable use cases and interactions. We then designed and implemented a prototype satisfying performers' common requirements. The prototype was assessed using the concurrent think-aloud protocol. On overall, the smart qualities of the prototype and their potential received positive feedback, and areas of improvements related to expressive control and personalization were highlighted.