Yes it’s free spinning, duct is about 6mm at the thickest spot. I can push it so it touches the prop but it takes quite som force. The question is wether the round hull around the prop deforms or the struts are too weak and bend. I think if I do as you suggested and cover just the tube inside and outside with glass or carbon it should be stiff enough. I guess its try and error.
Or print it in different material like petg or some carbon reinforced material.