The 'wet' model uses dry ice and 'dirt' and sand. A dry model would probably look like a large, 3-6 inch styrofoam ball, hacked up a bit so that it doesn't look round. Remember, Halley's comet nucleus looks like an Idaho potato! Next, you would have to dirty the ice ball some what by coating it with Elmer's glue, dipping it like a caramel apple!, and then roll it in sand to represent the dirt and dust trapped in the ice. Next, you carefully scrape away some of this coating to expose the cleaner ice, and to make the nucleus more reflective because cometary nuclei still have rather high reflectivity.
You may also add large trenches and spots here and there where you can have tufts of cotton stuck on to represent the jets and plumes of ice being ejected into space. There are usually dozens of big and small ones by the time the comet is near the Sun. Finally, you need the tail. This can be a large stiff piece of white construction paper which is shaped like a comet tail, and whose head has a hole cut out to fit over the styrofoam nucleus you just made. Then color the tail to make it look like streamers are coming out of the nucleus ball. You might even glue on some cotton between the styrofoam ball and the paper tail to show streamers ejected by the nucleus and entering the tail.