In the grand schema of things this is a very low power system and I wouldn't bother taking any precautions. Electronics will normally work quite haply in an ambient of 50 C, most chips are rated to work at 70 C which translates to a junction temperature of 100 C.
Depending on the ambient the box is in it might pay to keep it in the shade as direct sunlight into a sealed plastic box can up the temperature a lot.
The regulators produce the most heat and you can reduce this by feeding them with about 7.5 V rather than 12, the extra power is just burned.
This setup defiantly does not need fans, and at most would need some small vent holes top and bottom.