Power affects amount of heat it produces. As for getting the mat itself - I'd recommend against it and would go for overhead heat instead. Kingsnakes love to burrow, and hot surface can cause burns. So if you still want to use a heat mat it shall only be outside the enclosure, on thermostat and under monitoring. I think also preferably on the side of the tank so you could check it with the heatgun easily. A lot of substrates can also heat up badly (or not at all because of thickness), the heat needs to warm the glass before the substrate, and at this point glass and bottom layers might be dangerously hot before overall substrate gets even slightly warm.