I found https://makezine.com/projects/thumb-piano/ and was inspired.
I bought a grounding bar, and gathered plastic paintbrushes of various sizes. Then I installed it in our Acoustic Shell.
By the end of the day the paintbrushes were all broken. I bet I’d have better luck with metal tines as they would vibrate louder than the soft twang the brushes gave. However, I am still trying to find a place that sells blue tempered spring steel or street sweeper bristles.