Tub heat up easily due to poor insulate value they do not hold heat well for the same reason. IMO they hold humidity well but are very hard to heat unless you have a 'snake' room that is around the 78-80 range.

There are enclosures that do better in cool rooms but are costly. Vision, boaphile, showcasecages, and a handful of others I'd avoid top opening as they have similar issues. It might be a good idea to wait set up a tub and when you snake gets closer to adulthood then start looking at better ones.

I am guessing you are not interested in heating a room? I get it I am also in a drafty place.

You might try using hard foam insulation tacked on the lid and some on the back. Remember you need ventilation too so keep some holes clear. The other thing that has worked for some is to build a small case around the tub figure bottom, sides and top enclosed some have had success with this type of set up with a fluorescent light in the back corner. They get warm but not hot so they make a great secondary heater to warm up air temps.