there is not going to be an easy fix to your problem. No Heat or AC and big temperature fluctuations will make it almost impossible to maintain a steady temp on your snake's housing. Personally, I think that should have been thought out before acquiring a snake.
I'm afraid you'll either have to purchase a small Heat/AC unit at least for the room the snake is in, or you should consider keeping the snake at somebody else's house. I can not see any other options or possibilities that make any sense and that are easily accomplished. I have heard of folks wrapping their tanks with ice blankets to bring high temps down, and wrapping them with heat tape to bring temps up. But even still the temps go up and down, require constant monitoring, and will prove to be a nightmare.