I've fixed that on my A class rudders by cutting a slot in the rudders from the top and inserting a length of shaped flat stainless steel with plenty of big holes in it where it is embedded in the rudder, that way when you glass it in the resin goes between the holes and firmly grips it. I used vinylester to glass in then used a thickened mixture to make the outside bit the right space. To do that after the vinylester has gone off (about 20 mins), tape plastic to the board, lay the board on a bench with a slight angle down to the repair, mask off the repair, bunch the plastic up to a bit more than the right height using tape, pour in your thickened vinylester. If you want to use it tomorrow leave your repair in the sun to go off, you will be able to wet and dry to shape in about an hour. For some reason sunshine guarantees the mixture going off and it's not just the heat. Make sure you have the exact size drill bit for your rudder arm hole as the stainless will give you a non slop hole.