The heating pad needs to go on the bottom of the tank, on one side, preferably with a hide spot on top of it. Then on the other side, have another IDENTICAL hide spot for the snake to go in, therefore they can choose which temperature is best, not based specifically upon their best feeling of safety.

Tin foil will do alright, but I'd recommend going to Walmart, or any place of the sort to get a digital thermometer/humidity check for $12 which will be much more accurate in determining how you're keeping your temps.

Personally, I have a mesh top yet still keep cool side temps between 82-84 and warm side between 88-92. It's just a matter of the room surroundings, etc