Three layers of foam and glass (and of course external layers of sheething) and strongspots for the rudder fittings is strong enough by far.
The problem is rather the shaping. As the internal layers of glass are stronger than the foam, you will have difficulties making an even leading edge. This would probably be easier if you used foam with high density, skipped the internal layers and used carbon cloth instead of glass.
I made rudders of of foam, epoxy and glass, but I had a mould - and I vacuum bagged them. They have lasted six seasons.
Good luck!