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The white Styrofoam insulation would be more than decent at reflecting, if it has a reflective facing even better but I wouldn't use aluminum as that will hold/absorb heat and radiate it onto the plants, I'd see if the insulation does that as well by testing it with the back of your hand once you think it's sat next to the light enough. If it's hot to your skin it might be wise to try another thing.

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