Keeping the rudder from falling off

My Daysailer II had a small plastic device on the transom to keep the rudder from coming out while under way. I had to remove it because it was broken and I'd like to put something in place so I don't lose my rudder.
I've seen other boats with a metal tab that you press in to put the rudder on and take it off and would ideally like to install something like that. Does anyone know what these things are called and where I can get them?
I've seen other boats with a metal tab that you press in to put the rudder on and take it off and would ideally like to install something like that. Does anyone know what these things are called and where I can get them?