The black polycarbonate absorbs pretty much all the radiation that hits it. However, the polycarbonate itself radiates long-wave I.R. (8 to 10 μm wavelengths, according to the article). The purpose of the aluminum sheet is to reflect this radiation back onto the polycarbonate.
You can order black polycarbonate from McMaster-Carr, if you're in the USA. If you're in another country, you may or may not be able to order from them.
Scroll down to where it says "opaque black" here:
A 2 foot x 2 foot, 1/4-inch thick sheet costs 140 $US.
Compare that to black acrylic, which comes in thinner sheets and is much cheaper. Scroll down to where it says "Colored sheets to order" here:
Also from the McMaster website:
operating temperature -40 to 200 F
softening temperature 290 to 315 F
operating temp -20 to 170 F
softening temp 196 to 239 F