We have been testing these new Advanced PWM Smart Controllers on our commercial installations and love them. We thought you might like the option as well.
They are certainly not cheap, but are far more precise and controllable than a standard inexpensive PWM. They are in the class of CCPWM (Constant Current PWM), which simply means they automatically modulate the pulse rate to keep the amperage at the pre-set level all day long. And in our experience, they do a darn good job at that!
Additionally, You have a float switch monitoring option (remote monitoring of your liquid levels), voltage sensing (set it to turn off if battery voltage goes too low), Trigger switch (which will not allow the PWM to power up unless engine is running), and the ability to handle input from 12-24 volt systems, while outputting 50-100 amps, depending on controller.
Just install the PWM near the HHO kit and batteries (dry location), and hook up the connections desired as well as the input from battery and output to HHO generator. Then connect the CAT5 cable to your LCD display/controller, and you are ready to turn it on and set your parameters. Easy and very dependable.