Iv tried to use an evap cooler and it very dry here. They def don't do it. I had a 400w going that's it and all the cooler did was add tons of humidity didt really cool at all.
This is a common issue, you need to make sure you are evacuating the moist air from the grow area and pulling in new dry air, otherwise you won't accomplish any cooling, just humidifiying. The issue this causes is carbon filters don't work terribly well with large amounts of humidity, so you have to get it and try things till you get it dialed in. I know of a grow where it works very well even into the high 90's outside.
I believe you could DIY one of these much cheaper, but i know a lot of people don't have the ability, tools, etc for this to work. So you should look into a "portable" or "shop" evap cooler on wheels. Master Cool makes a great model, but again if you don't properly evacuate the moist air you are just going to fight yourself.