I made the special thermostat housing tool.
To start with I used a 3" (75mm) piece of 1/2" (12.5mm) solid round silver steel.
I then bored a 11/32" (8.5mm) hole into the 3" rod to a depth of 1 5/8" (41mm) using a lathe.
Whilst it was in the lathe I turned the outside diameter down to 13/32" (10.2mm) over the tubular end to a length of 1 1/4" (31.8mm).
Next using a junior hacksaw and some needle files I fashioned some saw teeth on the tubular end.
All that remained was to harden and temper the cutter end using the gas torch.
In my case I used the compressed air gun to clean out the hole, although a vacuum cleaner would most likely also work.