This works with vinyl - should works with CD's, they're heat-injection molded polycarbonate.
Set the oven on a low temperature, and rest your cd on a temperature resistant positive mold. (like a cylinder supporting the outer edges of the cd)
Heat in oven, keep an eye out for the plastic softening. Once it is bowing slightly, pull it out, and press the cd into the shape you want.
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Thanks to everyone with their suggestions! Sorry that I didn't answer sooner, I was away. This is the greatest site!
It may be challenging, the heat could cause the layers of your CD to split apart.
Try it outside with a heat gun. The best suggestion for a form to mould it on is clay or mud.
Make a form out of something non-melty (wood springs to mind). Use a hairdryer, or a heat gun if you have one, to heat the CD until it softens (not dripping soft, flexible soft), and let it ooze into the form.