Heating suggestions for cooler end?

[Sevvy]

Heya. So I've been heating my milk snake's enclosure with a heat mat on the warm end, but for the cooler end I've just been heating up the entire room since it makes everything easier. However, I'm looking at getting a second reptile and I'm going to need more nuanced heating control. Is belly heat still the best for terrestrial snakes on the cool end, or should I work more on raising the ambient temperatures in the cage like I've been doing?