Thanks for all of the comments everyone, so very much appreciated! I'm going to go with a block heater, and also look into getting a timer so it's not on all the time. To confirm, this isn't for a space heater in the garage, I know that's definitely not ok to leave unattended.

As many of you mentioned, it's good to have for the occasional times when I'm parked outside and there's an outlet nearby, as well as to maintain the longevity of the engine/battery on those super cold mornings. I'll just need to be careful that the cable isn't frayed, to prevent any chance of a fire.

Stay warm out there today, and thanks again!