An external device such as this shield by MIT has a number of advantages over securing the devices themselves.

FORBES: Protecting Pacemakers From Hackers

To protect electronics infrastructure, entire systems must be encased in a metallic shield to prevent any external electromagnetic pulse from entering.

CENTERFORSECURITYPOLICY: Old threat with a new face

To protect electronics, an entire system must be encased in a metallic shield that prevents any external electromagnetic pulse from entering.

CENTERFORSECURITYPOLICY: Center For Security Policy