... and I mean really poor man

First of all I wish to thank king.oslo and all posters of this recent thread bout 100V DC reference:
https://www.eevblog.com/forum/projects-designs-and-technical-stuff/100vdc-reference-circuit/Also Conrad Hoffman for his series of articles about building calibration devices.
I'm considering to build the instruments to transfer the accuracy of a 10V reference to
the usual DC ranges of a multimeter: .1V 1V 100V and 1000V.
Also I'd like to make the same with a reference resistor of some known value (10kohm? dont know yet)
As for AC no idea yet.
Why?
Learning excercise, learn to trust the meter.
Now my situation is to have a 7 and half multimeter that seems to function properly,
for some reason it lost the cal constants, I changed the eeprom memory and battery
but don't know if I can trust it for maintaining the calibration.
I have no cheap way to calibrate it.
It is the solartron 7061 and is relatively easy to calibrate, having the proper equipment,
and knowing how to do (I think it's not simple to measure things under ppm levels
correctly):
the points of interest are Zero, Low and High of each range,
where Low is 0.075 to 0.21 of range and High is anything from 0.75 to 2.1 range.
DC is calibrated Zero first and after High for each range,
for example 10V is calibrated first shorting the leads and making zero, after connecting a
voltage between 7.5 and 21V and inputting the proper value with keyboard.
Ohms asks only High values, only the highest range asks for open input leads, so no
problem.
For AC the only difference is that instead of zero the instrument asks for a Low range,
so for example 10Vac is calibrated using first a voltage of 0,75 to 2,1V and after a voltage
of 7,5 to 21V.
I could try to ask some friend at university or at some electronics industry to see if we
can organize a test to calibrate the instrument, comparing to some calibrated intrument,
but probably this will be a one off tentative, no possibility to repeat easily, and I dont't
know if they will have any stable voltage source to be used to transfer the accuracy between
the instruments.
Also my lab isn't thermally controlled, so probably there is no meaning to have the instrument calibrated
to last digits.
My main goal is to try to do the best I can with simple low cost and eventually scrap material.