This is an ultra-simple design of feedthrough (well, I think so!) which is rated at 40kV.
It uses a length of glass tubing as the insulator. The tubing is 9mm OD and 6mm ID. I first turned a piece of stainless steel rod down to fit smoothly inside the tubing. I threaded each end M4x0.7mm for the nuts.
There is a tiny little O-ring which seals on the inside of the glass tube. The metal cap at the left is glued to the outside of the tube and a 2mm roll pin goes through the metal cap and stainless rod to hold it in position and prevent it turning.
The glass tube is sealed to the vacuum chamber by another O-ring which is held in place by an aluminium flange and 3 screws.
A modification would be to accurately grind and polish the end of the glass tube flat so an o-ring could seal there instead of inside the tube. Since I had to cut a groove in the conductor, it limits the current which could flow.