Before someone tells you you don't really need a rudder, blah blah blah, you could have alook at the following - it has some drawings that may help web page
Well, when ah say 'plank', it were really more o' a piece of auld broomstick wi' a spoon fer a paddle. Took us across t'Irish sea and back though, and we were happy wi' it.
It looks like the rudder has a pintle /axle/hinge at the top and one about half way down.
you would start with a stainless steel shaft up to the tiller and have pintle attachment points welded to it to match the ships pintles/gudgeons. You also get webs welded to the shaft going aft and possibly down wards. The shaft should esxtend to about 3/4 the depth of the rudder with webs horozontal and the shape of the camber of the rudder. ie out to the sides front and back in a tear drop shape (out as far as the inside of the f/g layers.)
ie like a rudder skeleton in SS.
the rudder itself could be made of foam either glued in between the webs or even mixed and cast in to the webs such that it is then sanded down to the shape and size you want. Fill the gap in the area between the hull and the pintles and extend the whole rudder as long as you wish or dare. Make sure you put as much rudder as possible infront of the shaft to give better balance so lighter helm load. You can extend the rudder forward in the area below the fixed keel part of the hull. You can go up to about 15% arera in front of the shaft for max balance.
When you reckon you have a good shape in foam then cover it all with multiple layers of fibreglass. Polyester resin is OK. But make the layers are multiple and thick for strength and long life. (min 1/4 inch thick.Obviously you need to sand down the foam to allow for the thickness of f/glass.
Finally sand and fill the glass to a smooth finish and paint with Antifouling paint.
I reckon you can design /build as good or better than the original designer. Larger rudder works better but is susceptible to damage and additional loads. Smaller is cheaper stronger and will give you less control of the boat. Thicker rudder especially forward give better resistance to stall but greater drag. Thin rudder less resistance to stall and less turning power but less drage.
I extended my rudder by another 2 inches some years back and it handles better especially at large heel angles.
good luck pM me for more detail.or search fibreglass building/ repairs. olewill