I've never used rtk to mold a Dr. Seuss hat (made mine out of cake) but I have used rtk to mold other things. I just wanted to say to be sure and crush your rice crispies very fine so that you will have a smoother finish to cover with fondant. I'm just assuming that you will be covering the hat with fondant
Anyway that is a little piece of advise I wish that I'd have known before I tried to do anything using rtk. If you don't crush them very fine before you make them you'll have a very rough surface.
How big of a hat are you going to make?