I'm sorry to hear you lost your rudder! Can you further describe what you lost? There is a M8 hex bolt that's about 75-80 mm long and partially threaded. It fixes through the bottom portion of your rudder stock. Did this bolt come undone and you lost both the rudder and the rudder stock?
If not, there should be a rope tied through the very top of your rudder. This rope is used to pull the rudder up, but it also prevents the rudder from sliding all the way down through the rudder stock due to the stopper knots.
Flying Gorilla has a fantastic 3D printed solution for holding his rudders, but I have a low tech solution: