I am in SoCal as well and have dealt with the same issue in the past when mine were in tanks. I went to Home Depot and got a roll of bubble wrap with the small bubbles. I put two layers on the back and both sides with the bubbles pointing in. Some people even put aluminum foil over that. It made a difference of a few degrees.
As far as a small heater - I recommend that you go to Costco and get the Bionaire ceramic heater. It is only 20 inches high and will swivel back and forth or stay stationary. It has a digital thermostat so you can set the desired temperature, as well as a remote control. It is UL listed and even has an auto-off safety in case it gets tipped over. I use one and it easily will keep a room warm.