Michael
I have not studied the temperature switch referenced above, but my expectation is it would be wired between the Waytex timer relay and the fan relay. It should be a simple matter and no additional relay would be needed.
Looking at the wiring schematic attached to my prior post, the temp switch would be placed between terminal 30 from the Waytex and the separate relay connected to the fans.
The effect would be that the fans would continue to run after shut down until the temperature dropped to the preset level or the timer ran out, whichever occurs first.
Note the diagram includes a dash mounted switch to turn it off and on, but once turned on it will not go off until the preset time expires after the ignition is turned off. This was to give one the option of not running the fans - perhaps on a short drive on a winter day. With the temperature switch in the circuit the dash mounted switch may be redundant.
I can't comment on the use of ISIS since I don't use it. As one analyzes the wiring, two things to keep in mind. First, the Waytex will need a power source that is always hot and not turned off by the ignition, as will the relay that sends power to the fan. Second, consider switching the ground circuit rather than the positive circuit, as noted in the schematic. It can often simplify the system.
Finally, note that I added a second relay (Painless) which was not strictly necessary since the Waytex will handle the current. But this was for a practical reason: the Waytex relay is under the dash and the fans are in the rear and I did not want a high current wire traveling that distance. Keep the high power circuits short locating the fans and relay close by.
Hope this makes sense