A handheld electronic musical instrument, named the BentoBox, was developed. The motivation was to develop aninstrument which one can easily carry around and play inmoments of free time, for example when riding public transportation or during short breaks at work. The device wasdesigned to enable quick learning by having various scalesprogrammed for different styles of music, and also beexpressive by having hand controlled timbral effects whichcan be manipulated while playing. Design analysis anditeration lead to a compact and ergonomic device. This paperfocuses on the ergonomic design process of the hardware.