I did the same but worse as I towed the boat about a mile with the rudder dragging on the ground (what's that funny noise ? ! ! ) Mine was missing about 6" so I made a former from 10mm ply and glassed that in having gouged a bit of the foam out of the middle of the sandwich. I then used plastic padding as a way to form a new gelcoat. It took quite a lot of shaping and rubbing down, but the result is good, and you can hardly see the join. I now hold the rudder blade up with extra rope for towing, and don't rely on the tiller cleat.
Good luck,
Charles Scott-Knox-Gore.
BR 20 "Augusta Thomasina"
