Exploration is an intrinsic element of designing and engaging with acoustic as well as digital musical instruments. This paper reports on the ongoing development of a virtual-acoustic instrument based on a physical model of a string coupled nonlinearly to a plate. The performer drives the model by tactile interaction with a string-board controller fitted with piezo-electric sensors. The string-plate model is formulated in a way that prioritises its parametric explorability. Where the roles of creating performance gestures and designing instruments are traditionally separated, such a design provides a continuum across these domains. The string-plate model, its real-time implementation, and the control interface are described, and the system is preliminarily evaluated through informal observations of how musicians engage with the system.