Yes sometimes it takes several tries to get a fixture to do it's job as desired.
The thermal calibrator was one of those.
I start with a simple temperature controlled aluminum box, took to long for the air to heat the probes, as the leads were outside the box, and air could escape through the opening.
Second fixture has aluminum heat sink with bent fins to make direct contact with the temperature probe chip, but due to the curved contact area, the position of the probes was very critical, and even with some thermal grease, was all over the place.
After some thought, we made a water bath, with balloons to keep the probes dry, clothespins to hold the probes in position, and a heater to get the bath up to the required 120F that I want to do the calibration at. Warm up with the small heater takes a while, but once up to temp, I turn off the heater, and the thermos holds the temp very well, with only a 1 degree drop every 5 minutes. With the heater off, and some stirring, the bath is a very uniform temperature, so we can calibrate the inlet and outlet probes to be at the exact same reading.
Since the probes are to measure the inlet to outlet temperature rise of the pack, it is important that they are matched.