Free Space VNA Calibration

I am trying to do some free space calibration to extract material data from unknown arbitrary material. I am struggling with the calibration part as the s parameters i am getting is not good enough to get proper value. Can anybody please suggest the calibration procedure?
I have E8361 keysight VNA. I have Type N short, open, load, through (50ohm) but no standard precise calkit.
For free space calibration, what should I do?

You can't use a SOLT calibration for free-space measurement. You need to perform some sort of TRL calibration (ignore the "six-port" title; it's for a two-port analyzer).

To measure material data, you'll have some kind of test fixture connecting to the VNA ports. It's not actually free space measurement. Free space measurement may be made with antennas, but then you need to know or determine the antenna parameters.

Actually I understand couple of mathematical modelling behind extracting material properties where S parameters are related with T, R mu, epsilon etc. So far I understand the phase matching is important from antenna to material under test. Obviously I need to use either TRL or TRM, or GRL. But I am not actually sure without sophisticated calibration mechanism how can I do that.