I don't have any experience with block heaters, they're quite uncommon in my country, but a former school colleague once told me about an old man from his hometown who used to put a simple device known here in Brazil as "rabo quente" (hot tail) in the radiator of an old Chevy truck with a Perkins 4-236 engine (it had been a factory option for some Brazilian Chevy trucks).
It looks like this:
Not really so safe, there is a shocking hazard, but safer versions with a shrouded resistence are available.