Did I remember this formula correctly? So, 30 KHz would be a wavelength of...10,000 m? So a mesh with openings spaced less than 10,000 m apart should protect/attentuate EMP? Did I do that right?

Well, in that case, practically any enclosed space made of some conductor, like a car or plane, would act as a Faraday cage against EMP. Wait, then we're right back where we started!