Can't see why it is off topic. Much of the calibration is about test jigs or extensions of the reference plane. See hash tag #test-jigs. BTW, hashtags are active in this group. BUT they can only be created by a person starting a topic. There are several links in the wiki to practical uses of the VNA, many have test jig examples. Look under other VNA and related links. Issues with jigs are often calibration and error reduction.
Starsekr