I saved our old microwave, to use as a faraday cage 😂
Connect the chassis ground to an Earth ground, but I’m dubious it would be effective without additional shielding (many gaps).
I studied ‘hardening’ decades ago; many factors present, including both proximity to the source and location in the pulse shadow.
Of interest:
https://www.fourmilab.ch/etexts/www/effects/eonw_11.pdf
http://www.futurescience.com/emp/E1-E2-E3.html
Electromagnetic Pulse: Protecting Your Components
https://blog.gunassociation.org/electromagnetic-pulse/
Is the ground connected to a driven ground rod?