People, people, think about it. We used to cool our shop with a car radiator.
We mounted the radiator in the window. Stick a garden hose in. and put a fan behind. The water needs to move through the radiator, so we would open up the petcock on the
bottom of the radiator just a bit, turn the hose on slow. Water doesn't need to be going fast, just a fast trickle. In a grow room, you could make something smaller with
a oil or tranny cooler, or the heater core out of a car, which is just a tiny radiator. Now don't get me wrong, they won't cool like an air conditioner, but they can drop temps substantially.
We could take our shop temps from almost unbearable to very comfortable. So, it can be done easily. Those portable room coolers at walmart for 75 - 100 dollars are the same thing, they are
evaporative coolers, they are not air conditioners. So, go ahead if you want, but I would not use a radiator because it would be too big. Tranny coolers can be had for 25 - 30 dollars. They are small enough that they can be mounted to the side of a grow box. Building shit is cool, have fun, and good luck if you go through with it.