how big is the room? I'd go with a cool tube like TANDY said, or for a ghetto temporary fix used frozen two liters. I've done this before on really hot days, but my box is also pretty small.
Not to butt in, but i saw a newish thread abt changing light hrs with minimal stress. You can jst add an hour to your timer and subtract one off of it. This way every day that goes by your lights will stay on 1 hour later and turn off 1 hour earlier. Eventually you will reach your desired time.