Fermenting in an outdoor fridge

Hello.

[All temps are F ]

I'm a new homebrewer and I currently ferment in a fridge with a temp controller in my uninsulated garage. I started in August and am only on my second batch. Everything was fine in the summer as the controller kept a good temperature. My problem now is that as fall comes in Virginia, the weather is going through some pretty big swings from night to day (and even a few days are below sixty). I have been eying my thermometer and it's consistently between 60-61 (air in fridge-not the beer itself), though I do not check it in the middle of the middle of the night. I am not terribly concerned about my current batch as it was in the bucket for 1.5-2 weeks before the temps started dropping (though I was planning on ramping up the temps to the higher end of the yeast's range).

My main concern is for the batch I am going to be brewing soon (tomorrow if everything else works out alright). The forecast is for 70-80 days but 45-60 nights. Is there anything I can do to make the outdoor fridge work, like a water bath to keep temps better overnight? Will a simple table lamp with a sixty watt bulb put on another controller and stuffed in the fridge work to keep it warm without a huge fire risk? Would that lightbulb risk damaging my beer if it's fermenting in a plastic bucket?
Or should I just bite the bullet and buy a minifridge for inside the house during the winter? I have no basement to hide stuff, and have received the OK for a single minifridge but no jury-rigged fermentation chambers made of plywood and foamboard, so I would be limited to one fermenter at a time during the cold months. On the plus side, I will be able to keep brewing when the temps really drop, but I want to use my 2-bucket capable fridge as long as I can.

Thanks in advance for any insight.

I use a homemade fermentation box-plywood with 2 inches of foam insulation cooled with a window AC and a 2 stage temp controller. It's in my uninsulated garage as well. Summertime temps hit 115 or higher in the summer, then drop 30-35 degrees every night.
Because of these daily temp swings I placed a thermowell in my conical fermenter so I can monitor the temp of the fermenting beer as opposed to the air around it. Your full fermenter is a large thermal mass that changes temp fairly slowly, not nearly as fast as the air around it. If you are really concerned you could put the buckets inside a tub full of water which will increase the thermal mass, but if that water gets cold your beer will also stay cold.
This past August I fermented a Kolsch at 57F internal temp while my garage was well over 100F, but the AC would only run 10 minutes per hour to maintain that temp. In the winter I hook a heater up inside the chamber because my garage will get down in the 20's at night but can get up in the 70's during the day.

I put a 40w lightbulb in mine when it starts to get colder in the garage. I even have a lamp with an auto-dim feature so that I can turn it up/down when needed.

Your controller isn't dual stage? Then U would need to get one of those.

I made this and am happy with it. I'd rather have a 2-stage controller, but I didn't know enough when I bought my 1-stage. Maybe it gives you and idea, maybe not. Cheers.

Thank you to everyone for your replies