would you mind telling a brief story of your LM399 premod and postmod?
This particular LM399 reference was my prototype built on perf board. The original version (pre-mod) used the LT1001 op-amp recommended by LT on the LM399 datasheet tp scale the output voltage to 10V. This voltage was adjusted using a 5K trimmer in series with 6.8K. The unit was housed in a cast aluminum enclosure and the LM399 was further enclosed by a small styrofoam cube. 15V was supplied using an LT3080ET precision regulator. The LT3080 was supplied from an 18V regulated wall-wart type supply. Schematic Attached.
The modifications consisted of replacing the op-amp with an OPA177FP (pin compatible), replacing the 5K pot with a 100ohm pot and 1.5K resistor in series and lining the aluminum enclosure in thin foam insulation. Of those mods I think the op-amp replacement is the most important, followed by replacing the pot with a much lower value. The fixed LTC resistors are all 15-20ppm parts, while the pot is rated at 100ppm/C. Better resistors would help, but the cost are also higher. Pots better than 100ppm are not easy to find or cheap.
I now have a PCB design for these references that works well. I have a couple of devices under test currently with this PCB. Schematic of latest version also attached.