This was actually very easy to make. I got a couple of cardboard boxes (the right size for my kids). Then I found pictures on the internet and printed them out in several pieces and glued them onto the boxes.
Very creative and adorable, not to mention thrifty! Good call on the plastic bags. I spent hours making a cardboard car costume one year when I was a child and was very disappointed when the rain destroyed it by the time I'd gotten to the end of the street. I believed I pouted the whole rest of the night because I had to wear my overalls and dad's train engineer hat :) A nice clear plastic bag would have saved the night!