So last night I experimented. I tucked them into an oversized styrofoam cooler along with two 20 oz hot water bottles and a thermometer and left it all outside overnight to weather the winter storm. Knowing that the expected low of 30 was not that bad, and that they are already starting to go dormant, I felt like it wasn't too risky.
When I woke up the next morning, it was 30 degrees and snowing hard, but a toasty 59 in the cooler! Too warm, but I was trying to gauge how warm I could keep it when it's a lot colder out. I closed the lid back up, and when I got home from work, it was just below 50 in there. The temps never rose above 33 outside and it was covered in snow. So now I am pretty confident my flytraps could do well in there, so long as I let it air out a few times a day when it's cold for extended periods. Later this week when the temps climb back up, I will just leave the lid open and let them soak up the sun. I'm pretty excited as this seems to be a great way to take advantage of our many above-freezing days while simultaneously shielding plants from the much colder temps (at least nightly). I doubt I'll even need hot water bottles most days. And the lid is quite deep, so it doesn't squash them. But they do still have almost all their summer leaves so they will shrink quite a bit as the weeks progress.
Just really surprised at how well this worked and wanted to share. Please let me know your thoughts.