After fiddling with many PWM and voltage controllers based on a timer circuit and potentiometer input, I decided to go back to basics to control my AC PWM CPU and Exhaust fans in my Solo.
I simply used a 3 position toggle switch and some 1N4002 diodes in series to reduce the fan's voltage. An RPM header allows me to route the yellow wire back to the motherboard for speed monitoring.
Toggle switch mounted out back
(The second toggle switch controls diode 6(CPU Fan); optional)