Having been through the loop "are my DMMs accurate" a few times, I think in your situation, and for (relative) peace of mind, I would just order one of
these from China for <£10 and sit back. If you homebrew a reference you have to take a lot of things into account to get it a lot better than these. Such as thermal stress, variation with battery voltage etc.
These modules use the AD584K. It is accurate to +/-10mV at 10V so down at one last significant digit on a 3.5 digit meter (9.99 - 10.01V).
If you need something at a few ppm then there are companies in the states offering much more accurate reference boards on Ebay at £40 or so.
Great fun to homebrew but would you not rather be spending the time on mending things?
my two-penn'orth anway...
Jeremy