You're not going to like this suggestion, but...
Rebuild the U rig. The inflow on the heater, with the temp sensor, needs to be above the heating unit. This implies that water to be heated comes in from the top, and flows out the bottom. That way when the water stops flowing, the warm water rises to the sensor and shut the heater off.
Another option is to buy one of those "screamer" things that makes the gawd-awful racket when it gets wet, and position it for where it needs to be when you have a bad water day.
Steve - 33g reef and a 180g planted in need of a re-scape.