I'd definitely start adding chilled/frozen water bottles to the reservoir for two reasons: 1. You'll likely run into bacteria/slime/fungus issues in the root zone if those temperatures continue (speaking of which, how do the roots look?). 2. Lowering the temperature around the roots will help the plant deal with the higher temperatures above the root zone (outdoors, plant roots usually stay pretty cool, even when it's 100+f in the blazing sun).
Short of that, you'll likely have to start throwing more money at the problem than you'd want.
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Sorry I saw the "rubbermaid setup" part and incorrectly assumed you were in DWC.