[smily]

I would cut out the rest of the header and make a box or shelf to fill void.
I have actually seen where folks add a wire screen shelf above this ridge for additional storage and I have seen it done with oak shelf rail with little banister or spokes, it looked like it was OEM.

I think you are asking for a big mess by truing to repair with fiberglass. If you are not experienced with fiberglass, it is a real headache.