I'm sorry but I must push back a bit. You may be fortunate enough to get an accurate one for a few bucks at Walmart but I wouldn't count on it. Their accuracy will be all over the place. There are a select few guaranteed accurate thermometers that won't break the bank but they don't come from Walmart nor for a few bucks.X2, you can get them for just a few bucks at Walmart
Just be careful you don't restrict ventilation. You can build walls around the incubator with cardboard. It won't restrict ventilation but it will retrain some warmth.Bundling up for the night!