Hey Welcome!
First of all, we are not sure which book and by who you have. Some of the info there could be very well be outdated. 3 Months seems like a long time, I would do it once a month at least, depending on what you are using. I use newspaper so I have to clean the whole thing out when they pee or defecate.
What substrate are you using? be aware that pine shavings is toxic to snakes and reptiles in general.
You can try bumping the temp to 92. But remember that "thinking" is too cold is not the same as being cold. You need an accurate way of reading temp, or your snake will pay the price.
You can cover most of the top. They dont require as much oxygen as mammals do.