I got a 40B for my BP but I got a heat mat that's too big. The size is 18 x 8 I think. I thought it would fit but there's a piece of plastic that goes across the bottom of the tank. I don't want to return it but the temperature won't stay high. I don't know what to do. Should I stick it to the side of the tank? I'm worried that if my BP extends himself across the tank it could burn him if his belly touches it.