Two choices might work for you.
Marine Tex epoxy (it's a white paste) or 3M High Strength Marine Filler (a fiber reinforced Polyester resin, green).
In both cases you can improve results by trapping the repair in some plastic foil, perhaps one that's a bit stiff, like the plastic window on some packaging or overhead projector foils. The ides is to present a smooth surface against which the filler can cure.
Neither of these needs to be painted; the polyester paste, being reinforced, is a bit stronger, the epoxy will glue and seal better (but given the shape, anything will "wedge" itself in the cracks, so that's not an issue). You might like something that looks white, if you can live with the difference between pure white (repair) and off white (rudder), you can leave it unfinished. The green filler might not be as easy on the eyes
(I used it on my center board, where I didn't have to see it).
Looks from the picture like there is some exposed "core" material, so perhaps the epoxy paste may have a slight advantage here in sealing. The advantage of the 3M filler is that it is really strong and will definitely work standing "proud" of the repair and that it cures faster. But, the color
Good luck.