Abstract: An apparatus including an assembly associated with a downhole tool configured to thermally isolate a thermally sensitive component. Components of the assembly include an external isolating vessel; a passive thermal barrier encased in the external isolating vessel; at least one electronic component housed within the downhole tool for monitoring geothermal well properties; and at least one thermally sensitive electronics carrier package positioned within the passive thermal barrier comprising thermally sensitive electronic components. The passive thermal barrier ideally comprises an additively manufactured layered labyrinthine shell structure, a plurality of polarly phased centralizers between shell layers and minimal centralizing contact points on its exterior shell, to minimize known heat transfer modes associated with the external isolating vessel.