This paper presents a practice-based case study that explores personalisation as a design methodology for embodied gestural instruments. Through a multi-year collaboration with a professional performer with physical disability, we employed research-through-design and co-design methods to iteratively develop a personalised instrument responsive to the performer’s unique movement style and creative vision. The system enabled real-time control of sound, stage lighting, and visualisations, and was ecologically validated in rehearsals and in an award-winning disability-led theatre production. Extending beyond the stage, we refined the instrument through workshops with young people with motor, sensory, and communication disabilities, demonstrating its adaptability to diverse bodies and creative practices. Rather than focusing on generalised solutions, this work advances methods for designing technologies that embrace difference, tailoring interaction to individual capabilities. We contribute to HCI research by articulating personalisation as a methodological approach to inclusive interaction design, expanding opportunities for creative expression among people with physical disability.