I decided to make another voltage reference, this time using the cheapest components that I could find on eBay or Aliexpress. I even ordered the enclosure from an Asian source. Here are the results after more than 1000hrs.

2019-07-14 after running for 2 hours at temperature

Meter params: HighZ, 100PLC, AZ on

Value Range
100.0004mV 100mV
1.000007V 1V
2.50014V 10V
9.9999995V 10V (last digit bouncing)

2019-07-21 after running for 168 hours at temperature

Meter params: HighZ, 100PLC, AZ on

Value Range
99.9912mV 100mV
1.000014V 1V
2.50001V 10V
10.00008V 10V 

2019-08-28 1096 hrs:

Value Range
99.9925mV 100mV  (79ppm)
0.999978V 1V (36ppm)
2.49998V 10V (64ppm)
10.00021V 10V  (21ppm)

There doesn't appear to be any significant difference.