Foam board (foamex or Sintra) 1mm thickness is great for getting smooth curves around the back and cheeks. Epoxy filler bonds really well to this so you can fill it and re-shape if necessary. For the keyslots I used 3mm Sintra, this was perfect for this job, the thickness was spot on.
just search for "boba fett helmet template" and you should find what you're looking for. wizardofflight has posted blueprints for just about every fett part you could imagine