When I owned a C 150 I made a rudder lock that was 2 pieces of orange painted light plywood about 6 inches wide and long enough to span from the leading edge of the vertical fin to the trailing edge of the rudder. They were lined with carpet and had a bolt at each end. The wide surface spread the load and it was pretty obvious so it would be hard to miss on the walk around.
As for whether or not it is needed, well all I can say is the Cessna parked beside mine had the rudder damaged in a wind storm. The repair was almost 3000 dollars
I find the argument that using a rudder lock could result in someone forgetting it unpersuasive. A prominent well marked external lock is hard to miss. Anyone who does is going to show plenty of earlier signs as to his/her unfitness to use your airplane before they do something that stupid and should already be weeded out.