How do I make a mold from a plastic container?
There are kits available for making your own custom molds from food-grade silicone. The finished molds can be used for various cooking purposes including candy making and baking.

The laboratory I work for has used products made by The Smooth-On Company for many years and they are of high quality:


http://www.smooth-on.com/Food-Safe-Material/c1387/index.html


Because you're starting out with a with a non-silicone mold, you would use it first to cast some other material such as plaster of paris. Then, use the silicone kit to make your actual baking mold from that.

This video shows a quick overview of the process:


https://www.youtube.com/watch?v=8HyVCVfZ_g8


It's not exactly a trivial undertaking, especially for making a mold big enough to bake a cake. With so many varieties of pre-made silicone bakeware on the market, it may or may not be worth the trouble. (unless the cat is truly Adorable!)