I'm guessing that the AC unit would cost a lot of energy and it's ability to get down to temp would depend on its BTUs. Given that the average sized AC can do a decent job of getting a large room from the 80s into the high sixties fairly effectively, I wouldn't discount this option if you don't mind the electricity bill.
The best option that costs nothing might be to brew something that can tolerate warm temps, such as a Saison.
You could also search out a second hand refrigerator and use it as a fermentation chamber, although your description of the barrel suggests that a chest freezer might be more suitable.
Yet another option would be to find an even larger barrel and use it to make a swamp cooler.
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