I have 4 animal plastics T10 cages for some of my snakes. 3 of them i use the zoo med heat rope and it keeps things nice and warm and one i am using heating pads. I was just wondering how many heating pads i need to be using. he has 2 that cover almost have of his cage but they keep things really warm. they are the 8x18 size heating pads Also i use aspen for my substrate should i be using something else like paper towels or reptile carpet so the heat can get threw better?