I got this idea because my room is on the 3rd floor and the ambient temperature is 85 degrees (F). This build isn't exactly a poor man's, but at the end of this post I will list what you could have done to make it even cheaper. I spent a little less than $50 on this. I didn't count the box fan because I already had it.
I went to Walmart and Lowes to gather some items.
The shopping list:
20ft 1/4" copper tubing ($18)
5ft 3/8" vinyl tubing ($2.50)
Cereal container ($3)
Submersible aquarium pump ($20)
Zip Ties ($4)
Plastic Fitting ($2)
I wanted to make sure the pump would be powerful enough. So I tested it in a sink.
I uncoiled the tubing and placed it on the back of the box fan. 20ft was a perfect fit. Don't bend the tubing too much or too fast as it might cave in on itself. I used the zip ties to secure the tubing onto the fan.
I hooked up the vinyl tubing to the pump. Then the other end to the fitting, and attached the copper tubing to the other end of the fitting. The pump just sits in the case, no tube on the other end is necessary. The other end of the copper tube dumps back into the case.
It works perfectly. It pumped water through the tubes and subsequently resulted in a cooler air flow from the box fan. You can add ice for cooler air.
I cut down the vinyl tube for a more refined look. Also, after a few days, the fitting started to fail a bit on the vinyl tubing part. I used duct tape and a zip tie to keep it tight and secure. So far it's been good to go with no further problems. The pump does generate noise. It is sort of like a low hum, you forget about it after a few minutes.
For a real poor man's A/C.
You can skimp on the cereal box and use a cut up 2 liter.
You can buy cheaper and smaller zip ties.
You might be able to get away w/o the fitting by using a lot of duct tape.
You can get a smaller amount of copper tubing. I believe 10ft was about $9 at Lowes.
You could even forgo the vinyl tubing; situate the copper tubing just right and you'll be fine.
Unfortunately for the pump, it was the cheapest I could find.