A fairly common method- use knox gelatin and jello molds. cut the lens, use it, eat it later.
I've wanted to do a 'build optics' day.
I've done the gelatin molds. The one I am missing is mirrors. I've read (Dr. Falco of Arizona University) that one can make mirrors by using two bars of metal and a paste btn the surfaces. Continued rubbing will make one side concave, the other convex. I don't know where to get the appropriate pastes.