Go from scratch.
This tutorial is what I used, except I used carboard rather then foam, and gesso rather tehn spackeling paste.
Hope this helps. If you wanna go my route and use carboard (you name make it more stiff) then paper mache eachsection like this:
Make the base
make ALL the parts that get glued on but DO NOT GLUE
mache EVERYTHING separate
then glue it all on..
then sand and paint.
It will look ten times better but that's only for the cardboard version..I mention this version because cardboard is free....