First, I hooked up 2-100 watt light bulbs and strategically placed them in the engine box. I connected them to a thermostat cube plug that comes on a 35F and off at 42F. I also picked up a Kestrel D1 Drop and hung it in the engine box. The Kestrel measures and logs the temperature and allows me to download the readings to my smartphone via Bluetooth. I watched it for a few cold nights and everything seemed to be working fine.
However, I still wasn't resting easy.
Would the two bulbs be able to keep it above freezing if/when the weather drops to single digits? What if one of the bulbs was to blow out? Murphy's Law would surely result in a failure of any kind occurring when I was out of town. What I wanted/needed was a backup plan. 
I looked around for a 2-stage thermostat but couldn't find one. So, I build one instead. Here's what I did...
First, I added two more light bulbs on a separate circuit.. The first circuit has 2-100 watt bulbs while the second circuit has 2- 150 Watt bulbs. That's a total of 500 Watts!
Next, I bought 2- STC1000 Controllers for 9 bucks each, an Ammo Box from Harbor Freight for 7 bucks and grabbed 2 duplex outlets from my leftover inventory. Mounting the STC-1000's in an Ammo Box wasn't my idea. I got it off of YouTube. What was my idea was to mount two of them so I could create a primary and secondary circuit. Next, I set the primary circuit to come on at 38F and off at 48F. So long as the primary circuit can hold the temperature at 38F or better, the secondary stays off. If the temp drops to 33F, the second controller turns on the second circuit.
The coldest temps so far have been in the low-teens' and it seems to be able to maintain the engine box well above freezing. I'm sleeping much better
