you can go the NMAS route and tape a piezo to it.
Yep, getcha a Barcus-Berry hot dot and some bonding goop and you're golden
These are small contact piezos.
May be called a Barcus Berry DISQOS now, they're pretty inexpensive.
A good hardwood washboard should transmit the sound fairly evenly.
A metal chestboard could be a bigger challenge, but experiment with placement and you'll nail it.