Back when Bowman were manufacturing, and modellers were making their own, the modeller would use what was available, usually using the TLAR principle. Celia May was designed with this in mind, allowing the modeller a free choice.
I would look in the fixings aisle in a big DIY shop for brass rod about 4mm diameter, and some tube that would be a sliding fit over it. "Thin" can have different interpretations for different people. 1mm is "thin" because it can be easily cut with tin snips, but can also be considered quite heavy in othe places. But a 1mm slot can be cut into a 4mm rod.
A bit out of the spirit, but ready made rudders are easily available these days.
TLAR = That Looks About Right