Best take to it with an angle grinder with sanding disc on it ( 40 grit) and cut away the void area making a hole with shallow tapered sides until you are well clear of the void area. Put a backing piece across the hole with kitchen wrap or similar on it as a parting agent, then lay in layers of glass cloth in epoxy until slightly over filled then fair back to the true surface.
I’ve often said that the problem that cant be fixed with epoxy and fiberglass cloth has yet to be found, thats not quite true but for sure it will fix a lot of things, your problem being one. Good luck with that.