[AC_Hacker]

I called a sign-supply shop and ordered some Coroplast, or more correctly the same product made under another name.

There was a difference in price with regard to color...

White was about $13 per 4 ft x 8 ft sheet.

Black was about $17 per 4 ft x 8 ft sheet.

I went with black even though it was a bit more expensive, because I think that carbon is probably the pigment used and is likely to improve thermal transfer.

I bought 2 4x8 sheets. Since I don't have a van or truck, I had them cut into 12" x 12" squares (slightly less because they use a saw and kerf allowances were needed).

There was a small charge for cutting.

I still have the problem of if and how to join the pieces...

-AC_Hacker