Too many variables. Too many thermometers and too many locations in the box. Eliminate the too many thermometers variable.
First, choose the thermometer of highest quality, one of the glass tube type, but not the one that came with your still air which has the degree index on a slip of paper. No. A good old fashioned, glass tube thermometer.
Then, bring your incubator up to temp. Have a couple of water drinking bottles filled with tepid water, approx body temperature, your body temperature. Place a couple of those in the incubator as heat sinks to simulate the bio mass of eggs.
Wait. Wait Wait. Wait at least 4 hours for the temperature to stabilize and begin taking a reading. The thing is that you're going to have to find the "mean" or average temperature in three or four places in your box. Back corner, front corner, middle, etc.
You'll likely find a 2 or 3 degree swing between those areas. That natural without a fan moving the air about.
You'll want to "average" about 101 on a still air unit and have no spots above 102. 103 will begin to kill eggs.
And good luck. Have patience. Don't rush the egg deployment until you feel it is safe to put them in.