Thank you so much, everyone, for all the helpful replies and well wishes!
I wish I had known about the heat pads before tonight. Unfortunately, I work later than my local store will be open tomorrow, and by Friday, the earliest time I could get to the pet store, the weather is supposed to be the worst here. It's looking like my best strategy will be wrapping blankets or towels around the 5-gallon tank if the power goes out, and if the temperature dips too far, putting him in a cup or his old bowl and finding a way to keep him warm that way. Thank you for the candle in a glass container suggestion, Aryia. I don't have a gas stove, so I might have to resort to that if I can't get to a store of any kind after getting off work tomorrow.
I'm having so much fun watching Rakki explore his new tank that I hate thinking about possibly having to take him out again so soon. But if it ends up being what keeps him safe, then it's worth it.
If any of you is facing this storm as well, I hope you and yours stay warm and safe!