Have done lots of simple probe calibration checks. It's all in how you make the ice slurry, which is simple:
Take enough (party) ice to twice fill thermos (or low thermal mass container).
Fold ice into rag or cloth to contain it, place on firm flat surface (floor) and beat it with mallet, hammer held sideways or some such. The aim is to smash it up as small as possible.
Fill thermos/container with smashed ice, including the fine 'snow', tamping it down.
Now add just enough water to make it look watery. The slurry should still be more stiff and solid than liquid at this stage. If you put in too much water, pour some out and pack in more ice.
Leave for 5minutes.
The water in the slurry will be 0'C (or close enough for Government work!)
We use this, along with a calibrated probe, for checking probes in situ to 1 decimal place.
It is very accurate and readily repeatable, provided the ice is smashed small, tamped down, and minimal water is added.
The calibrated probe tells us when the slurry is going off (rising in temp, which happens after approx 30minutes and multiple 6mm SS Pt100 probes), it can then be renewed by pouring off some water, adding more ice, stirring and leaving for 5min.