Are your room temps cool because you live in a cold climate, or because you like living at that those temps? In my snake room I keep the door shut and the heat vents open in winter, which ups the temps. In the summer I close the vents to prevent the air conditioning from coming in there, and natural sunlight heats the room nicely. Of course not having a dedicated snake room makes that impossible, if you don't want to live yourself at reptile ambient temps. If you use a space heater be sure to be ultra careful. Those things cause tens of thousands of house fires yearly. Oil heaters are much safer. Ideally you would use a space heater that has an internal thermostat, or you can hook it up to an external thermostat. Like others said, limiting ventilation on the actual tank itself will prevent heat loss to the exterior. If you can afford it, any pvc enclosure will be better than a glass tank. However, glass tanks do perfectly fine for the first couple years of a boa's life. Melamine or tinfoil on top will help prevent heat loss from the mesh top. I believe there is a tutorial in this forum to show you how. You can also play around with having multiple different wattages of heat bulbs to achieve the correct temps. For example, you might have a 100w infrared bulb for the hot spot, and then a 75 or 50 watt bulb offset from that one to just up ambient temps, but not really make a hot spot. Its weird to think of a heat bulb being used for a 'cold spot' but it does work. Also, don't get too crazy wrapped up into getting ambients at a perfect 80 deg. If your boa has a constant hot spot, and ambients are at least in the 70s you are doing fine. Much of the newer ideas coming from top herpers is that we have all been keeping our reptiles far too hot.

Try to get the basics set up, and then watch your animal for cues. Right now my ambient temps in my room are at 80-81. My adult boas have a 90 deg hot spot constantly. However, my two biggest females rarely are on the hot side. Even if I lower the hot spot down to around 87-88 they still don't spend a ton of time over there. Point is, even if you have your temps exactly 'ideal,' your animal might not even use it, so don't stress minor fluctuations.