I recently built a rocket from stuff around the house as a prototype for a future project of mine. I used some left over sculpting foam for my tip (sculpts with just sandpaper; love that stuff; super fine stuff). Resin over that to give it strength and sanded it down again for paint.
The expanding foam in a can could also be used. Just shoot it into a form block, shape it down; Bondo it perhaps instead of the resin, sand and paint.
Styrofoam could also work but it will take a bit more attention to get smooth since the texture of that stuff tends not to be smooth. But like above; Bondo, sand, paint..
Just off the top of my head. Hope you are inspired to your own conclusion.
-If I get around to it, I will post a few pics of my build of the foam tip.