Using a Zoo Med ReptiTherm UTH, the small or medium one, but would need to measure to be sure.
Used a plastic tub because that's what I've been reading works well, better than tanks according to many. We are planning to build a snake rack, using melamine, which I've read should help hold in heat? I have a feeling we'll be adding to our snake collection in the future, in which case large enclosures will be impractical (already have the rosy boas set up in a large, divided tank, and it does take up a lot of space).
Sounds like one of two things:
1) I can try using a localized heat source, such as a heat lamp.
2) I'll have to heat up the room.
Two of our bedrooms are newer construction (2006 I think, compared to the rest of the house which is 1940 or older), and better insulated, so it probably won't be a huge issue to heat up one of those. I typically end up turning down the heater at night in cooler weather (and during hot summers I need to turn up the A/C and/or run a fan at night) because the bedrooms do stay so much warmer than the rest of the house (including the hallway). Guess I'll experiment a little and see what works!