If you are making a 3d model of a cell it is important to be able to see inside so some clear plastic is needed for the cell wall. Maybe a clear ball that you cut open allowing you to put in the inner parts of the cell and then taped closed. You will need to construct a nucleus, mitochondria, ribosomes, lysosomes, and Golgi complex. Find a website that has pictures of what these look like and then imitate them. You could use macaroni for the mitochondria and a pipe cleaner for the Golgi complex.