We special-ordered a reel (4000 pieces) of NTC caps from Capax. They are 0805, 3.3 pF, N4700. 23 cents each.
I figured I should test them. I made an oven out of a roll of solder, with the cap and a thermocouple inside.
It has a nice long thermal time constant. I started with a heat gun to get it to 50C, covered it with a cloth, and took data slowly as it cooled. Near room temp, a spritz of freeze spray into the solder coil cooled it down, and then I took data as it slowly warmed up.
The measured tempco is close to -7000, but that's OK. I'm going to compensate a 600 MHz coaxial ceramic resonator oscillator, with this cap and a series NPO padder, so too-high tempco is OK.
It would be great to have a very small benchtop oven, peltier hot/cold, to test small parts or circuits.