A high wattage hairdryer will work just fine. I wouldnt put a book on top just for the fear of any texture from the book cover transferring over to the plastic. Just pop a piece of glass out of one of your picture frames and use that, assuming its not a huge plastic.

The only danger would come from overheating them in the oven and as long as you keep an eye on them that won't happen either. Just make sure your plastic is clean and you are using clean surfaces and don't worry - its easy and safe.