Thermostat probe placement incubator question

Going to build a incubator out of a cooler. My question is do I tape the thermostat probe to the heat tape like in a rack or do I let the probe hang in the air to regulate to the air temp?

air temperature actually inside the tubs is what you really want (even though mine is hanging in the air)

Mine is zip tied to a shelf right in the middle of the incubator. When getting it up and running, I adjusted the thermostat temp based on the temp in the tubs (which in mine aren't far apart at all).
http://uploads.tapatalk-cdn.com/2016...735497faeb.jpg

My temps will fluctuate as much as 1 degree between tub temp and air temp outside the tubs. I am currently using a cooler that can hold up to six egg boxes. No fan. Lots of water bottles to help stabilization of temps. I am actually fighting the opposite problem than most. I have to keep temps down. Without AC my ambient can hit 89, so AC is set to kick on at 88.

Thanks everyone.