While mixed reality has inspired the development of many new musical instruments, few approaches explore the potential of mobile setups. We present a new musical interaction concept, called "opportunistic music". It allows musicians to recreate a hardware musical controller using any objects of their immediate environment. This approach benefits from the physical properties of real objects for controlling music. Our prototype is based on a stereo-vision tracking system associated with FSR sensors. It allows musicians to define and to interact with opportunistic tangible widgets. Linking these widgets with sound processes allows the interactive creation of musical pieces, where musicians get inspiration from the surrounding environment.