Well, since a piezo has a direct pressure response to vibrations, you should just be able to connect a spring across the long end of your box, or along the underside of the neck, maybe. Sympathetic vibes from the strings and the resulting air waves bouncing around the inside of the box will set the spring singing, which will be picked up by the piezo. Pick's Swamp Witches have an internal reverb spring that works this way.
Hmmm. Now you got me thinking: if you slung a spring inside a piece of metal conduit inside the body, or even the neck, you might get a really kewl tube wave generating an interesting reverb effect. Hmmm.
yup .. only by request now, do i include the spring .. mine don't use a piezo , but the vibes from the body will echo it through the sound hole acoustically .. and a mag pup can also reed it , more-so if the (steel) spring is close to the pup.
Simple reverb/whammy: L shaped rod, 3 or 4 holes along base of L aligned with upright of L, small 2 hole pipe straps or oversized holes for screws to hold L in place (oversized holes allow rocking motion) as tailpiece, string tension returns part to neutral position after pushing or pulling to bend and torque those notes and chords! Open up the L to your preferred hand position for use before final assembly.
I did a reverb box in a suitcase where an amplified pizzo is used to excite the spring and a cheap bucker is used to send the movement of the spring back to an output, works great when u kick it, just like an old twin