The probe is outside the enclosure on the heat tape itself. I have it run like a rack: under the bottom enclosures, up the side, under the top enclosures. I have the tstat set to 90. Where the heat tape goes up the last part of the wall, near where the wires connect it, I have it taped up against the side. I taped the tstat probe there on the heat tape and then taped the heat tape against the side of the enclosure.

I used some "strong" duct tape, but apparently the heat made it less sticky. I've put in the order for the foil tape and will use that for everything going forward.