My reef tank would have a hard time in the summer before I bought a chiller. My AC was a piece of junk and didn’t work half the time.
Before the chiller, I did two DIY options.
First I bought a plastic pickle jar sold at Wal-Mart and a couple of bags of blue ice. I filled up 3 Gatorade bottles with the blue ice “stuff” and siliconed the lids shut. They would fit perfectly inside the opening of the plastic jar. I would then add silicone tubes for in out and tie it in with my filtration. I would keep all 3 bottles in the freezer and switch one out everyday.
Next was a mini-fridge, which worked great till the compressor went out. Was also nice to have my own fridge to keep those icky fish foods in that the wife despised being in our regular fridge.
In the end I bought a real chiller and it has been in operation for about 5 years now. I think DIY are fine for short-term solutions, but in the long run, actual chillers are built for the job and are the best choice.
Luckily, my Planted tank stays between 80-81 so chances are I wont need a chiller for it this summer.