I would go with the single door forget the water heater thermostat and find a old fish tank heater and take it apart and use it has worked really well for me.
This is my simple version that can be used for a automatic coop door or a egg turner.
parts list
1 timer to start cycle
1 time delay relay to cut power to nc side of microswitch nc stands for normal closed.
1 microswitch to stop motor on no side of microswitch on 180 deg intervals for...