I would get rid of the bungee system on the rudders and put a pull up system using a line. You need a couple of Harken 404 blocks, CL-211-AN clam cleats, Rope balls and I use 4mm Robline 8-plait line. I can send you a couple pic's if you want to see how it's done.

Problem with the shock cord system, If your rudders hit something and kick up you have no steering. I don't care what cord you use it starts stretching almost immediately. Then they sag down and drag on the beach etc. With the pull up line you put the rudders up and they stay up.

For the pull down line I use a three strand 1/4" twisted Nylon. You can find it at west marine. (the only thing I buy for my cat there) The advantage of this line is it stretches and the plastic cleats hold it well. If you get a line that doesn't have any stretch in it what will happen is this, as the line seats into the cleat it will move backward a bit. This allows the rudder to kick back a bit which affects the helm.