I wasn't sure how it would work either, I bought the thermometer at Walmart, (about $10) and check the temp everyday. It runs about 20* warmer inside compared to outside. We have had some low 20* nights here already and lots of ice, and it stayed above 45* the whole time.
I added the bricks and sand to the bottom so retain heat better. The little electric heater takes the chill out of the air. It has blower so the plants
get used to the wind.
I say just try it, buy a thermometer and check it everyday. The real trick is trying it keep it from getting to hot. I leave for work at 5:30 and don't get home until 3:00. I go out in the morning before I leave and slid the window glass up a little if I think it will be to hot, and go out at night and shut it down.
I have the thermometer hanging from a nail in the very top so I can check the hottest spot.
I looked at the plastic ones and didn't really like them. I had most of the material to build this one.
Try it and keep some records of what it is doing, and adjust what needs to be adjusted.
I will try to get some pictures of this years seedlings
I hope this helps,