I wanted to try thermoforming a CD, so I did a little Googling and came up with a great instructional page at http://www.plasticsmachining.com/magazine/199609/polycarbonate.html#.
Based on these instructions, my first experiment was to place a CD--aluminum layer up--into a small Pyrex mixing bowl so that it rested horizontally, suspended by only the outer circumference of the disk against the inside of the bowl, at about 3/8-inch from the bottom of the bowl.
After removing the bowl from the oven and allowing it to cool, I was able to easily pop the disk out of the bottom to inspect it. The aluminum had become slightly wrinkled in places, where it had separated from the disk as the plastic deformed. The plastic itself looked fine, except for where it had rested directly on the bottom of the bowl; there it had obtained ring of slightly rough texture, probably caused by tiny bubbles in the plastic. I don't yet know if this was due to the extended heat, or to moisture in the plastic, or to the fact that it is a fairly old Pyrex bowl and the bottom is somewhat scratched.
My next experiment will be to score the aluminum layer in a radiating pattern with a razor blade and to heat the disk with the aluminum layer down. My hope is that the scoring will allow the aluminum to separate slightly so that it will not inhibit the movement of the plastic so much.