Don't use lids.
You can buy Corrugated plastic sheet. They are about 1/4 inch, so they should replace the lids. You can either screw them in place or glue them, but try it first before sticking it in place.
For the probe, put in the middle shelf (let's say your second row on top or bottom), right on the heat tape between 2 tub. You can use sillicon or something to keep it in place. Then you adjust your thermostat to fit the temperature inside your tub. Let's say you have a 2.5 degree difference between the temperature on the heat tub and inside your tubs and want a 92F temperature, put your thermostat at 94.5F. But you will to test it before getting the right temp you want. Also, it's should take a few hours for the rack to stabilize at the right temp, since the melamine need to take some heat etc...
good luck
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