Hi Tom, When it comes to this forum, I am the "defender of the old rudder".
I have a 71 boat, and have never had a problem with it releasing prematurely. I was out in 20+ mph gusts with 20 degree shifts this past Sunday and again, no problems. Here is my advice on having that same experience.
1. Make sure the rudder tube is in place and in good condition. That tube prevents side to side motion of the rudder, and if it's missing the rudder is going to blow out of the bottom bronze strap. Mine had deteriorated to the point where it was falling apart. You can search this forum for the dimensions.
2. As Webfoot1 touches on, I replaced the bronze screw that holds the strap to the bottom of the boat. I believe it's a #12 bronze screw, but I cannot remember how long it is. I filled the hole with epoxy and then drilled a new hole so that the new screw would hold better.
3. Also as webfoot1 suggests, make sure the bottom strap is in good condition side to side. Mine looks perfect, and I have never seen one that wasn't, but then again I don't see a whole lot of 71 and older Sunfish. Who does? If yours is damaged, however, I would search for one on eBay.
4. I have the wing nut tight, but I wouldn't say it's at max tightness
5. I think about the boat being balanced. Dragging the rudder through the water sideways puts a lot of stress on the rudder, but more importantly it's really slow. When I was out Sunday I slid the lower boom out to 20" or so and pulled the outhauls tighter than usual. I always make sure I am not over-sheeted.
5. When I launch, I usually just drop the rudder into place and put the pin in with the rudder down.
Hope that helps.