An authentic Neapolatin oven uses traditional materials such as S. Maria brick, Sorento clay floors, Tuff, Vesuvial sand, sea salt, etc. But you're not building an authentic Neapolatin oven either. So you can use ceramic blanket or perlite as an insulator. For ceramic blanket, 1" is barely adequate, 2 or 3" is a lot better. 4" is ideal and results in an exterior that is barely warm to the touch. Don't forget to insulate under the cooking floor too. And the lightweight IFB would work for the exterior dome of your oven. Have a look at the Forno Bravo oven building site if you aren't familiar with it.