I couldn't get the dome to shape up like I wanted so I set up the dome base with the two cross pieced and glued in styrofoam as filler. I then filled in the remaining shape with Crayola brand air-drying clay. After the clay was dry I just fiberglassed over the form.
If you go this route, make sure you undersize the form by the thickness of the fiberglass, otherwise you'll have an ill fit to deal with. And I would suggest covering the form with something other than aluminum foil as a resist. Perhaps plastic wrap and then dry fiberglass cloth.