In the 2002 WMPC, five boats had serious rudder problems, and one boat was abandoned. Here is a simple design for a backup rudder that is light, small, and cheap.
A common rule of thumb is that the backup rudder should be at least half as big as the original rudder. This should be big enough to steer the boat. You offset the smaller size by deploying the backup rudder abaft the normal rudder and possibly by steering harder.
Our design is set out here. The rudder attaches to a spar, and we stabilize the spar with guys. We limit stress on the rudder by steering it by its trailing edge. The rudder is therefore supported around its perimeter. This permits a lighter, slimmer, and less expensive design overall.
The major role of the spar is to hold the rudder down. Since the guys take up the side-to-side stress, the only critical load on the spar is upward compression. A 2x4 or spinnaker pole will easily handle the job.
See the pdf for full details