Incubator Temperature Question.

So, im doing an incubator using a container of water with an aquarium water heater & a Styrofoam box.

so i place the water heater at one end of the box. The heater is in a container of water and for several days now, i slowly adjust the temperture by 0.5 degree C. (i'm using Celsius instead of Fahrenheit, like what most of you guys are doing)

i started adjusting the heater from 28 C, now its at 30 C. And i place another thermometer at the other end of the box, now after 2 days, with the heater set at 30. im getting the other thermometer reading at between 30 C & 30.5 C averaging two days already.

Now i got a few question.
The 2nd thermometer is place on the other end of the box. Measuring the temperature in the box. Do i need to put another thermometer in the egg box? As i think the temp in the egg box, will not be the same as the temp in the Styrofoam box, or will it equalise after a few days. I havent place an egg box in yet..
Do i need to get the temp in the egg box at 30 C or everything is ok already?