n-player
Why don't you just buy a pink foam insulation sheet at the local hardware store, like that used in the Newer Hampshire TechIngredients videos?
That cheap foam will be easy to mount and move around transducers and you can ignore any foam damage.
I seem to recall the TechIngredients video illustrated the basic impact of moving the transducers around and adding more transducers with sand. That might provide some basic transducer guidelines for this application.
You can cut the foam roughly shaped to your piano using a saw or maybe a heated metal string. Then mount the foam behind the piano. If you were using a M&H grand piano, for example, rather than hang the foam from the bottom of the piano just cut some wood straps for legs like a table; the legs may have some audio impact (positive and negative) but are good enough for testing and you could move them around.