Craig,
Here is a very simple approach to an "adjustable rudder control" that is
used on a Southwick "Skylark".
By being disassembled in the photo, the required parts are illustrated.
After fabricating the plywood disc, mount and glue it into the recess, and
then sand flush with the fin.
This same system can be used underneath the stab.
These were the parts that I choose, but you can do something similar
with sightly different parts.
And don't waste your best hinges in this application, because they are
just not needed. These hinges don't even have a pivot wire...they just
flex in the middle of the hinge. This provides a bit of 'stiffness' in the
hinge which may reduce minor vibration of the rudder. I don't remember
what brand they are.
Cheers, and stay safe.
Warren Wagner
AMA 1385