I had to fix ours up not long ago... This is how I did it:
1. Take 5 or so strips of oregon (glued & clamped) - enough to make a plank about 12" wide by 3/4-1" thick.
2. Plane to shape roughly by hand, then use 40-grit sandpaper to smooth and final shaping.
3. One coat of epoxy with one thickness of glass mat laid up whilst still wet.
4. Final coat of epoxy over the mat when dry.
5. Sand out any imperfections, prime and paint.
Using a soft wood makes it easy to shape the blade, whilst epoxy coating gives it strength - true, you don't need
to glass it, but if the wood is a bit thin in places it helps. You could use straight glass or carbon fibre, but you'd have to mould it somehow and, unless you're after really light weight, if you're only making one rudder that doens't make much sense to me.
You could use a polystyrene foam sheet, cut it to size, sand it and glass it, but making the pivot hole strong enough might be an issue and later reshaping is impossible (foam isn't as forgiving as wood).
Good luck with your project.