How can I use the wood knocking notation in the strings part?

I'm going to write a long piece called "Concerto for Principle Players and Full Orchestra." In the middle of the 1st movement that I'm creating, The Solo Strings and the harp are knocking on the wood of their instruments. And I need to find a way to add that wood knocking notation and find a sound of the knocking on the wood of the instruments.

Any ideas?

Thanks.