I'd do 1.5inch thick and brace everything, 3 layers on the front baffle, with the third layer I usually flush mount. It's still adding stiffness and looks a lot better IMO. If you do that, flexing is kept to a minimum while still leaving a lot of airspace available. You don't see them a lot, but window bracing is particularly strong and flex resistant in my experience. It also couples perpendicular walls together, which I feel is a large benefit.