Oh that's a big tank hon. You might want to downsize that a bit. Much easier to heat a smaller tank (way less volume of air which is hard to heat in the first place as air doesn't retain heat like a solid does) and your BP likely doesn't need anywhere near that space if it's a little one. How big is the snake?

Tin foil shiney side down can reflect both heat and humidity but please do not use duct tape or any tape anywhere near a snake. Even on the outside. Accidents can happen, a bit of tape gets caught under the lid and you have a badly injured or dead snake. Saran Wrap is just naturally slightly sticky and I believe they now sell a self-sealing type Saran Wrap that would be quite effective (just trim to tightly fit the lid and not into the tank whatsoever).