If you have the chance to add a swamp cooler if you live in a place with low humidity, your chameleons may thank you for it.
If you can't afford that, try adding something to the sides.
Remember, and aquarium does not have enough ventilation. Products like the Protean enclosures, Exo Terra, Zoo Med, and the like, have sufficient ventilation.
However, the trade off is the amount of money you have to pay for good ventilation.