My rudder idea for the Sea racer looks like the attached image (perhaps not everybody in the thread has access to the plans ;-)
The idea was to have an extremely light and simple but hydrodynamically efficient solution (retracted or employed), with few parts and put together witout fasteners of any kind and therefore serviceable anywhere.

The rudder blade hangs on a pin, held in place in the shaft. The cylinder is kept in place by the two-part uphaul, tightened in front of the cockpit with a piece of bungy cord. The cylinder top fits into a square hole in the line sheave, that lies loosely in the top compartment of the shaft and connected to the rudder lines.

All parts are manufactured from glassfiber and epoxy, and as molds were used standard plastic plumbing pipes of different sizes. The cylinder is foam-filled to be light.
Everything can be assembled/disassembled in a minute without tools: just release the bungy cord, drop the rudder cylinder and remove the pin...

The comment on being not autoretractable is of course valid – unless the rudder happens to be in neutral position. But the prototype rudder is controlled by a another new (?) idea – a pedal set made out of 3 mm plywood cut to produce two pedal plates with rudder lines attached to outer upper corners. These "pedals" conform to the shape of the foot when bent – and they flex back to neutral when released, meaning that the rudder always will be in neutral and retractable (by the paddler or by a rock) immidiately when not engaged.

The comment on rounded corners are also valid – I did that after testing – but not too much, since it creates turbulence!