A ply wood doubler of some type with epoxy is more than enough to support it. I have use popsicle sticks in a pinch to support the sides of a rebuild like that. If you want to use fiberglass use resin. It is easier to sand. That area is one of the most common areas to break on a stick. A little more support inside the fuselage will not hurt it. If it was me I would add some ply with epoxy on the inside and call it good.