Stealthy and simple but not efficient. If you want a designer solution, I would did increase the heat capacity of the exposed surfaces by increasing the mass of the walls. Sounds: hard, expensive and needless? yes, what did you expect from an engineered solution. But it's neat.
Materials:
Two fish tanks of different sizes (both are big)
Industrial adhesive or chalk
Spray paint
Water proof aluminum foil duct tape
Pump
Water cooler
Tubing
Pump
Water with anti-bacteria agent (you can use something crazy like anti-freeze)
Water proof aluminum foil duct tape
Glue one tank inside the other.
Spray paint the inside of the inner tank (tanks where the plants go)
Add send and receive tubing
Place the water cooler at the same shelf level as the tank
Install the pump and tubing to the water cooler
Add Solution
Tape up the the space between the top opens of the two tanks.
As the lights warm the walls and the solution, that heat is stored in the water cool. No more 500 watt heater.
