I used something like this: http://www.homedepot.com/Building-Ma...&storeId=10051
I'm not sure if that's the exact one, but the sheeting I bought is styrofoam with a reflective silver side. I cut it to size (very messy - styro gets everywhere!), aluminum-foil taped the edges for aesthetics, and then spray-painted the white side black (leave the clear plastic film on when cutting and taping, otherwise the paint just gets sucked into the styro). Let dry well, then attach to rack with reflective side facing inward.
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