I would set the heater stat only a bit hotter than the external stat. Then if the latter fails and is always-on, the temp will only go up a few degrees, and you are likely to notice that if you monitor,
I posted these pix on the CDR thread the other day, but folks here might appreciate how I enclosed my LL Reptile thermostat that controls both my Kane heat pad and overhead Radiant Heat Pad.
Harbor Freight Ammo Box - drilled out hole for extension cords and