I've designed a 3D printed rudder system for my newly acquired AE convertible kayak. All the files for printing your own, along with a part list and instructions are posted at: http://www.thingiverse.com/thing:820015
I am very much a novice kayaker, and any feedback would be great. (BTW, this has only been tested in very calm conditions - I wouldn't recommend relying on it in rough water without a lot more testing.)