NO NO NO NO NO NO NO NO NO NO NO AND NO !!!!!!!
YOU SHOULDN'T DO THAT !!!!!
I have never incubated yet but I know this is WRONG !
Physic 101 : Heat move and temperature fluctuate. and mass retain energy (heat in that case).
By placing the probe IN the eggbox, you ask the thermostat to adjust the temperature inside the eggbox.
While, if you place the probe OUTSIDE the eggbox, the thermostat will adjust the temperature of the AIR SURROUNDING the eggbox to 89, making the eggbox to 89.
You will have LESS FLUCTUATION in temperature INSIDE the eggbox by placing the probe OUTSIDE the containers.
It's like placing a glass of water in a pot of water on the stove. Measure the temperature inside the glass and in the pot and play with the stove. If you put the temperature to "HIGH" you will see that the glass of water will take more time to get to the same temperature than the one in the pot, but then, if you close the heat, it will remain warmer than the outside, then you put back the temp on, and the outside will get warmer and the glass of water won't drop as much.
Same apply to an incubator.