I changed the plans and ran 3 rudders instead of two. I don't think it is worth the extra work. I once did something stupid and lost 2 of the 3 rudders and had to get back 8 miles down a narrow winding river with all 3 of my kids and myself. We did fine with one rudder and a fully loaded craft.
See below for steering and throttle information.
I also changed the steering from the plans because I did not like the cable on top of the deck area. I built a shaft of 3/4" conduit and ran it all the way to the rear of the craft. Then I welded a piece of 1" flat iron onto the rear (see picture). This flat iron is bent back slightly to keep the cable in line with travel if it is not in line you can get some slack. To mount the pulley's I used a 6" piece of 1" dowel, and drilled into the craft and epoxied it. It is rock solid. The cable is run to the 1"flat aluminum that keeps the rudders in line. I used a small turnbuckle to take the slack out of the wire. I attached the handle in front by following the plans and drill and taping a large bolt. By drilling and taping you reduce slack.
This steering turned out very well except...The tolerances are very tight between the wood motor mount and motor pulley. It is also to tight where the idler pulleys change direction for the thrust.
The best thing I found for a throttle handle was a mountain bike brake handle. It has the throw you need to open the throttle all the way. I ran bicycle brake line to the motor.