This is the window cooling unit I designed about 2 years ago and how I am doing mine:
A: Fan (placed in window to intake cool air)
B: Mount for junction of fan and duct tube.
C: Flexible aluminum air duct (wrap in layers of duct tape to act as insulation and keep the air colder longer before it reaches your terrarium.)
D: Ultrasonic Humidifier
E: Output of humidifier stuffed with a hose that intersects the air duct BEFORE it reaches the growing chamber
F: Growing Chamber
This setup can be used wherever you live, but to rely on outside air you must have cool temperatures. In warmer areas simply put the intake fan in front of an airconditioning unit. So in warmer areas you can have highlanders, you'll just pay for it continually whereas I only need the AC through July and August to get my nights cold.
A timer is not as accurate to run this setup as is a humidistat (to just control the humidifier). The fan is benefitial to run 24/7. Just take it out of the window (or just shut your window) during the day so temps in the terrarium can warm up.