There is no guidance on where to drill a hole...
Below the bottom rudder hinge the fin spar doesn't do very much. There is plenty of space above the bottom bolt for a hole.
Consider leading the wires vertically so rudder deflection twists the wire bundle rather than bending the wires.
In the rudder it is difficult to secure the wires - lay them in the glass fairing inside some snakeskin.
Also consider how the light will be disconnected in the future if the rudder has to be removed. I ended up with a terminal block on the rear deck and wires long enough to reach it. The type of connectors chosen will dictate the size of the hole in the fin spar.