White "wiffle balls" would perform better as they don't retain the heat, they are white to reflect sun, not black absorbing heat, and the slots allow air exchange at night to cool water. And more importantly they are commercially available at a reasonable cost.

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White "wiffle balls" would perform better as they don't retain the heat, they are white to reflect sun, not black absorbing heat, and the slots allow air exchange at night to cool water. And more importantly they are commercially available at a reasonable cost.