How many gl is your reservoir? I use 18 gl of water in my reservoir and my temps were getting 80+, i dont really have the option for a chiller because of room reasons, not to mention ive heard they make the area really hot... So what i did was take 2 one gallon milk jugs rinsed them out and cut the tops off and freezed them, just alternate the two milk jugs, my temps have consistently stayed from 64-72.. Big blocks of ice take longer to melt down then cubes, the plastic on the milk jug is thick enough that it wont shock the roots, just make sure you have the tops cut off so the ice can melt into the actual reservoir, Then later that day fill the carton back up with water from your reservoir and put in the other frozen one... Depending on your ventilation you might want to do 3 frozen milk jugs and alternate ever 8 hours or so..
Hope this helps..