Would there ever be a need to shut it off before the timer ends?
How are you actually controlling the heater?
I posted a thread awhile ago to control a coffee pot (I’m still using it) you might be able to adapt
- it has a count down timer displayed
- a button on the dashboard
- it turns a smart plug on/off
I’m the button turn it on for 20 minutes and - this is something you would need to change - the button can shut it off too.
In your case I would have a second button to add more time so the original button still could shut it off if needed.