I'm not talking about a rock, it's more like a bulb. This is an example:
https://www.amazon.com/Zoo-Med-Repti...rds=che+heater
Is the thermometer in the middle of the tank the only thermometer you have? It's important to have a digital thermometer on the inside of your tank, under your coco right next to the heat mat. This is the maximum temperature your snake can feel. There are numerous stories on this forum of snakes getting burned because heat mats are being used unregulated by a thermostat. Here's a decent starter thermostat:
https://www.amazon.com/MTPRTC-Contro...art+thermostat
It's also helpful to get a more accurate thermostat/hygrometer. This will be closer to the floor, where your snake is and give you a better idea of the humidity and air temperature:
https://www.amazon.com/AcuRite-00891...ter+with+probe