I am using the following thermal cut-off by the heaters, since I am going to enclose the two tubes in a box.
(I am using Mouser.com as the supplier)
Per Heating tube.
364 Degree F Thermal Cutoff, 526-NTE8181. Install this on the hot leg of the heater.
High temp Heat shring tubing, 4.75mm dia. 650-CJ6775-00
I will post pictures as these come in and I build.
I want to keep the temperature external to the insulation well below the ignition temperature of wood
I selected 364 Degrees F on the outside of the insulation by the pipe as a not to exceed temperature, and it would be a temperature
which would suggest a problem with the system (no flow etc). 364 Degrees F should never occur in normal operation, so I will not be swapping
thermal fuses every brew cycle.
You could select a more conservative thermal rating, 250 degrees F if you are not enclosing the heaters, or if want a greater safety margin.