Mobile interaction is shifting from a single device to simultaneous interaction with ensembles of devices such as phones, tablets, or watches. Spatially-aware cross-device interaction between mobile devices typically requires a fixed tracking infrastructure, which limits mobility. In this paper, we present SenseBelt - a sensing belt that enhances existing mobile interactions and enables low-cost, ad hoc sensing of cross-device gestures and interactions. SenseBelt enables proxemic interactions between people and their personal devices. SenseBelt also supports cross-device interaction between personal devices and stationary devices, such as public displays. We discuss the design and implementation of SenseBelt together with possible applications. With an initial evaluation, we provide insights into the benefits and drawbacks of a belt-worn mediating sensor to support cross-device interactions.