Hmn... throw an HPS in there. That will warm things up. ;_)
What is the lights off temp, 55f? If this is the case, I would just add a couple of more lights to get the lights on temp up and the lights will never hurt. If it is colder, I would select a heater which will also produce some CO2 for you in the process.
Keep an eye at your RH levels, as a heater will often dry things out a bit.
I am concerned about ventilation with your setup. You need to have fresh air coming into your box and air flow through the plants for healthy growth. What are your plans for ventilation?