The purpose of this article was to develop a custom voluntary-closing, body-powered thumb mechanism for a partial-hand amputee who had amputations of the index finger and thumb on the left, non-dominant hand. Case description and methods: The prosthesis model was manufactured using rapid prototype technology and was developed to provide greater force and functionality, and to decrease overall size compared to traditional hand prostheses. Findings and outcomes: Following device iterations and occupational therapy sessions, the patient achieved higher functionality in performing daily tasks such as cooking and cleaning, and in completing the Box and Blocks test, though some limitations still precluded full acceptance of the device. CONCLUSION: This case study represents a unique approach in the development of custom-made devices that may increase prostheses acceptance rates among partial-hand amputees. Clinical relevance Many partial-hand amputees report experiencing trouble in finding a device that fits their needs. This study highlights the potential of using rapid prototyping technology to design a prosthesis that meets a user's specific desires.