I have a 40 gallon fishtank (I know, I know, not the ideal enclosure, but its what I have for now) and a 21'', 127gram ball. I am apline substrate with an under-cage heater (for a 40-50) gal tank, and it covers about 1/3 of the bottom. I have one temperature gage on the hot side (I will be getting a second one for the other side) and it is reading at about 75 (room temperature). I know this is low, so i put a heating lamp to warm it up, and now it registers around 79-81 depending on the day. For now I put a towel on top of the lid to keep heat in.

My question is this; What can i do to raise and keep the temperature around 80-90* (this is what i have read is the best temp range for ball pythons) without having to use the lamp (bc I BP dont need "basking heat") Also, is having the lamp on for now going to be okay? Is there a certain time i should turn it off (to simulate "night"). Is there a more efficient way to keep the heat from escaping the top besides a towel?
I am new to this whole deal, and while I have been researching for the last couple months before I bought the little guy, I still feel like I am helpless! Any additional advice is most welcome.

Thanks!