The aluminum is very nice for this application because of what it does for the box above - it provides a path for the heat energy through the metal. Use a width that exceeds the flexwatt width by some. I tried 6-inch flashing over 3-inch Flexwatt, and it produced a heated area ranging from very warm in the center area to less warm on the edges. Nice effect.

The hardest part is having the aluminum lay flat, being sold in rolls and wanting to roll up. I didn't want to glue it down permanently, so I used some aluminum tape in spots. After some use, the aluminum stayed flat on its own.

I looked online for some 1/16 thick aluminum sheet. I found some pre-cut in squares. Cost is up there as you imagine, so I didn't follow through with that.

Heating vinyl to herp-related temps is not dangerous as far as fire is concerned. After all, Flexwatt is coated in Mylar, also a soft plastic. However, heating vinyl may release chemicals that you would rather not have in an enclosed area like a rack. Use the aluminum, or purchase Mylar sheets for additional protection.
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Mark

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