Check out pwmpower.com for free help on how to build your own pwm unit. These units can change the frequency, voltage, etc. of the power to your hho cell. This will allow more HHO production with less heat build-up. Heat is your enemy. Search pwmpower on youtube.com and check out his videos that will show you step by step how to build your own. He has gone out of his way to help those that have little money build their own.
He also sells PWMs for a very reasonable price. If you are lookntfnd HHO cells, check his channel for references.