Make the Probe
The easiest way to make the probe is to insert a cable
(preferably a silicone cable) in a nipple that fits your boiler. The casting can be hard silicone rubber or reinforced epoxy that can withstand over 100 degrees. See Figures 2 and 4.
Another way is to fit a teflon tube into a bush with a brass shaft. See Figures 1, 2 and 3. The fitting must be tough. (The teflon expands in heat and moisture which makes it dense)
Tips & Ideas
Sometimes it can be difficult to fit the probe because of too few boiler bushes in the boiler. One way is through the water gauge upper nipple. See Figures 1 and 2.
Another way is to enter a blanking plug. See Figures 3 and 4.