I've gotten this far with a small electric heater here in MN. The forecast is calling for subzero temps and even my heater won't be able to counter that. I could insulate but I don't think the cost to continue heating is worth it.
I have a Harbor Freight 10x12 and use a small electric oil heater. With good daytime heating and minimal opening, I can keep mine at least 10-15 degrees above the outside temp. That’s been good enough for container peppers and tomatoes.