How about cutting the resistor's leads off completely and just
clamping it between say two 1" cube copper blocks? But then I guess
you would still have the problem of connecting it to the VNA. :-(
--
Would possibly work OK if your standards were fabricated in the same way.
When it makes a measureable difference whether you're measuring to the front
face of the SMA center pin and the back of that same pin, you can start to
develop an appreciation for the accuracy possible with the VNA. I haven't
measured the capacity between two copper faces one inch square and separated
by the length of a quarter watt resistor, but it would be appreciable. (and
measureable if I were really interested.)

W4ZCB