Yep, that's the way Popsie.
The fogging is caused by the reaction, vaguely mentioned in my first reply. Acetate, and PET (Polyethelene tablet) have a different composition to straight polystyrene, and can exhibit clouding, discolouration, and shrinking. It can also turn brittle much more quickly than 'styrene, particularly in sunlight or a strong UV light source.
Although costing a little more, it is better to use 'styrene, and of a slightly thicker gauge than possibly required, as styrene, rather than shrinking with the application of heat, will stretch slightly. This actually allows the approximate thickness required, as the 'stretching' takes the form of thinning of the sheet.
Whichever material is used, the application of heat needs to be done carefully, and normally takes practice. Even then, I expect a success rate of one in three, at best, 50%, from each moulding attempt.
I've found that, although it is possible to make the male mould from materials such as silicone or 'Milliput', it is better to use balsa wood, with the grain filled and polished, using a mixture of gloss varnish and talcum powder. When enough coats are applied, with the penulitimate coat lightly sanded and a further coat of clear varnish applied and polished when hard, this minimizes any reaction that might cause fogging or the 'white' reaction. I believe that most of this is caused by a reaction with the alternative materials, which doesn't seem to happen with the 'traditional' balsa mould. The latter also last a very, very long time - I still have some made over 30 years ago.