Customizable systems that let children and adults with disabilities control audio playback can support different forms of therapy, including music therapy and speech-language therapy. We present SenseBox, a low-cost, open-source, customizable hardware/software prototyping platform to turn everyday objects into audio triggers for people with disabilities. Users can add tags to physical objects that when in proximity to SenseBox trigger the playback of associated audio files. We designed SenseBox with input from three therapists and an assistive technology expert. We detail our human-centered design process that took place over 16 months and describe a detailed example use case where SenseBox was used to create an accessible music player for a child with cognitive disabilities. This project illustrates how to design physical computing prototyping platforms for therapists to create customized interfaces for their clients without requiring prior programming or design experience.