Hello J -- welcome to the Forums! The answer is 2". The semi-monocoque stiffness comes via the contribution of both the inner & outer skins. Don't discard that inner skin! At least you'll use it as a pattern. The design is "semi" monocoque because the frame & floor aren't an integral part of the shell top-to-bottom like an aircraft fuselage would be.
Various foam insulation schemes have failed due to slight shifting movement in the shell grinds most to dust. High insulation isn't the complete answer due to the lack of a thermal break with the metal ribs. I could imagine various ways of overcoming that while still maintaining the structural contribution of the inner skin.
High enthusiasm isn't the complete answer -- see http://www.airforums.com/forums/f44/...ign-38897.html
. Best wishes in your project -- we'd still welcome working with you if you'll open your soul (Danger Will Robinson!). The forum format yields incomplete and too frequently abrasive answers. I'll certainly wish you a shop full of metal working equipment and experienced first-hand assistance.