Can chickens float out in a thermos? What other methods are there?

Hot water bottles are not allowed. . . In order to hatch chicks, the temperature needs to be constant. How can I keep it constant with a thermos? I checked and asked for a constant temperature between 38 degrees Celsius and 42 degrees Celsius. So, of course, you can play in the incubator. I can make a simple incubator, involving a little electronics knowledge. So if you don't know anything about electronics, you may have to buy it yourself. Incubators sold on Taobao cost hundreds of dollars each.