The LM399 get quite hot inside - this is rather high for the OP and even resistors would see increased drift at that temperature (e.g. 90 C). So it is better to keep the OP cold and maybe have the resistors somewhere in between. If really needed for stability (high TC resistors) add an extra oven / control loop for the resistors and the ref chip with it's cap and maybe even extra insulation.
Having to much heat conducted away from the LM399 will also cause trouble with the internal heater, as it might work to hard and cause internal gradients.
I would avoid potting, as stress can cause extra drift an the OP and resistors. More like a closed case with some glass-fibers inside if too large.