A/C (air-conditioner) pulls heat away, making the inside cold. The fan sucks air through the A/C, making the air cold. The warmer it gets, the more effective this A/C gets.
And it's easy on your wallet!
The materials are:
-popsicle sticks (about 20)
-cooling fan from computer
-solar panel, batteries, or other low voltage power source
-cloth that absorbs well
-hot glue and a glue gun
Step 1: Cooling fan... Meet the popsicle sticks.
Once you find a suitable one (bigger ther better), stick six popsicle stick like the picture below.
You can trim the round ends with scissors if you want.