Restrict the airflow on the mesh lid. Don't make it airtight, but restrict it enough to trap heat without causing condensation. My cube has a front opening door that allows plentyof of air in- I just don't want to lose heat too quickly out the top, as mine is warmed by the lights from above.
I have a smaller cube with a small light above, and a very small heat pad below. A marble slab slightly larger than the heat pad diffuses the heat and allows a more gradual drop in temperatures at night for younger/smaller snakes.

Just watch your snakes. If the spend all their time at one end or extreme, adjust. If they stay in the mid range, or move about with the time of day, you are probably good.