Finally made use of this info today and prototyped a temp controlled keg for fermentation.
I discovered that not only does the probe fit in a standard dip tube, but the lead cable fits thru the poppet valve hole in a ball lock gas post. This makes building a thermowell really straight forward.
I installed a flat oring inside the gas post which fit the ID of the gas post and the OD of the probe cable. When then post is tightened this compresses the oring and forms a seal. A modified dip tube becomes the thermowell.
I set the SP on the A419 for the temp I want the wort, but I also put an older manual Johnson control in series w the A419 till I get the differential set right. The probe for the manual tstat is inside the freezer and is set to the lowest temp I want in the freezer. This prevents getting the freezer too cold before the wort temp drops.
A few pics attached.