Not sure why this is downvoted. Carving away styrofoam is a great way to see what the real world form of something will be, especially considering there's a good chance you can fish it out of your trash. And, a hot glue gun mean you can put pieces back!

For getting an idea, I like to play with clay, then move to CAD once something is fairly decided, with foam/cardboard mockups to get "real world" size feel. Then, a desk full of 3d printed mini versions, and then whatever final version.