I recently ran across a 'formula' for making a synthetic comet nucleus on the Internet during a search for information on comets, but I did not write down the web page address where I found it. I think that you can create your own version of this 'formula' that is just as meaningful using the following reasoning:
Comet material is thought to be a mixture of various ices, and grains of dust made from graphite and silicate. You need to get some ordinary sand, and some graphite. The graphite could be expensive to produce, but you could simulate it with Nestle's Cocoa! Take a mixture of, say, two cups of water, and add about 1/4 cup of sand and cocoa. Pour this into an ice cube tray. To help you see this material better, add some food coloring to the water to give it a light tint, but not so much that you cannot see the dark cocoa in the water!
The major constituents are water ice and carbon dioxide ice ( dry ice). But for simplicity, just use regular water ice from your freezer. For realism, you should take a number of ice cubes and break them up into a variety of sizes.
Now you want to simulate how the ice in the comet might be clumped together by adding your colored 'dirty' ice cubes, to another container like a plastic food container, and add clear water to cover almost all the ice chips. Freeze this for a while until hard. I think that the recipe I have seen calls for Karo Syrup instead of the clear water, but the effect is the same.
What you would now have is a clump of ice in which you can see chunks of colored, dirty ice suspended. This represents the clumpy appearance of many comets which were formed by accumulating other pieces of ice, but never getting hot enough for the clumps of new material to blend into the rest of the comet. The ice is dirty because interplanetary space is filled with small pieces of rock and asteroidal debris that will get swept up by the comet as it moves.