... or use the existing vertical fin along with the two fins you've shown at the tips of the horizontal stabilizer.
Yep, or like the Boeing 314 which started with a single vertical stabilizer/rudder, poor control response,
so they tried a twin vertical stabilizer/rudder layout. Still not right, so finally they went with the twin
vertical stabilzer/rudders and a centre vertical stabilizer (no rudder) with the same shape and area as
the original single stabilizer/rudder.
So mebbe keep the standard stabilizer/rudder but fill the line between the fin and rudder and add
the new outboard stabilizer/rudder assemblies.