Clean little setup you've got there.
Your house is too cold, turn the heat up!
I would probably get a little portable heat and aim it towards the intake fan. Put it on a timer, so its running when the lights are off. It will take a little tweaking to learn what setting to put the heater on and where to place it outside the case.
Another option could be to get one of those reptile heat mats and put it in the case. Turn it on opposite the light schedule. Couldn't tell you how much this would raise the temps though....
Never had experience with this, but those are what I might try.