If you go to Home Depot, they have sheet of styrofoam which can be cut to fit a specific surface. Replace the sunshade with a sheet of styrofoam and use the towel over that if there's a reflection. . . or, line the inside of the trunk with the styrofoam, make an enlarged cooler of the entire trunk area.
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Better is the Enemy of Good Enough.
Okay, what’s your point??