The plans and the instructions do not explain what the bottom rib of the rudder is made of. The plans show some dotted lines that hint how it is made.
Don’t do what I did and make it from RS-17, that would be wrong.
The bottom of the rudder rib is made just like the bottom of the fin as detailed in section C-C on drawing 10. It is built up from RS-8 and RS-1.