I used a 1/2 wide slice of fiberglass insulation centered on foil tape to insulate the probe to the pipe and furnace over the load door. Then I used magnets to help hold the foil tape down. Works great! My reason for insulating the probes is so that any stray air currents in the basement can't influence the temperature I'm trying to read, it acts as a blanket..
Disregard the snap switch mounted on the pipe, it runs my draft inducer...
Just to be clear, top to bottom is tape, insulation, probe, then pipe or furnace.
Make sure there is nothing between the probe and the pipe or furnace.