I own an electric smoker and have a pid controller for it that I bought at auber instrument
. From what I understand the pid controller works a little differently than the Ranco/Johnson controllers with a tighter temp control which might be overkill for a fridge. I have not looked to see if they have lower temp controllers (the prebuilt smoker controller has a min temp far above what you would want for a fridge)
Auber's customer service is great (I emailed on Saturday of July 4th weekend and received an email back in 45 minutes asking for my phone number to troubleshoot my problem) -- I would email to see if they have what you want prebuilt or custom.