Well, I'm not much of a cook so custom cakes and such are out of my league. I am, however, an aircraft mechanic, so I do understand sheet metal fabrication and templates. So what I can imagine is, take a piece of cardboard or poster board, draw an outline of a big rat and cut it out. Bake a rectangle shape cake. Lay the template on top of it and cut away everything that doesn't belong. Cover the remaing rat shaped cake in frosting and such. Enjoy.
That is my idea. I'm not even sure if it would work, but it seems logical...
Rob