I don't like to be obvious - but why not put a timer on the heater itself?, or even better put a thermostat on the heater (which it should have anyway).
With a proper thermostat you don't need to do silly things like 'put it on for an hour', just switch it ON and forget about it - once it gets to temperature it will turn itself OFF, and then come back ON occasionally to maintain the temperature - oviously it needs to be well insulated, but again it should be anyway.
I ran a immersion heater like this for decades, (as millions do) and never had big bills.