If they don't have to be clear, I would recommend food grade silicone. More durable and easier to demold though a bit expensive. Chef Rubber is a retail supplier.
If I had to guess, I'd say that there's a good chance that the molds made out of white tempered plastic have a fair chance of being polystyrene. I'm not sure but a quick check of the melting point indicates that it shouldn't melt at that temperature.