if you have an open-top viv, like me, it can help a LOT to throw something over the screen top, like a sweatshirt, or a sheet of plastic, that will hold heat in to increase ambient temp. I think if the room is 68, we'll be hard pressed to keep an open-top viv at 85 in any part, and 75 may be tough when the lights go out.
This may also increase humidity, I'm not sure.
My sister has done this for her anole (pet, not food, lol) and it kept plenty warm. I don't think that it'll hold enough air in that the snake will not have enough oxygen.
I may also put a small electric heater in my snakes room. does anyone with experience have an opinion?