When putting up or taking down sails or just motoring around (for shame), I use my "Tiller Jock" to control the tiller and rudder. Although I bought this device, it could easily be made from some shock (bungee) cord and some round wooden balls from a craft store.
The "Tiller Jock" is basically two equal lengths of shock cord (about 2/3 the width of the stern) and two round wooden balls. Although mine has a clip on one end of each shock cord, you could tie a loop in one end of each shock cord to go around the cleats.
On the one that I have, the wooden balls are 1" in diameter with a 1/2 inch hole in the middle. The shock cords are a 1/4 inch in size and approximately 4 1/2 feet long. After you have drilled the 1/2 inch hole in the middle of the wooden balls, you thread the two shock cords through each hole in the two wooden balls. This will allow you to make a loop in the middle between the two balls. Then tie a knot on one end of each of the shock cords, one to the right of the balls on one cord and one to the left of the balls on the other cord. This will make sure they do not slip back through the holes.
As I said, I have a clip on the other ends of my cords from the knots, but you can tie a loop (a bowline would be good) on the other ends of each of the cords.
To set up my tiller jock, I loop one end of each of the shock cords (see above) to each of my stern cleats. Then I put my tiller through the loop between the two wooden balls. (see right picture). Then holding the two shock cords together, I slide one ball next to the tiller. Then I do the the same to the other wooden ball. As you can see from the pictures, I leave a little slack. If I need to move the tiller more to one side or the other, I can slide the balls accordingly.
The advantage of using the shock cord is that if I have to take control and override the tiller jock, it has enough stretch so that I can steer the boat. When you are finished changing your course, just release the tiller and it will return to the original position.
When you want to "disengage" the Tiller Jock, just move the two balls away from the tiller to the ends of the lines (knots) and you will have plenty of room to steer the boat with the tiller without using the shock cord to hold it in place.