If you have access to welding equipment and a lathe you might consider making your own HV feedthrough out of a spark plug.
A champion N3C spark plug when cut open has a 5/8" (15.66mm) OD alumina core. The core is easily good for 50KV, been there done that. In hindsight next time I would cast the HV electrical connector using a 2 part silicone rubber.viewtopic.php?f=10&t=4237
The alumina core diameter should be fine for 80KV if the plug was instead sealed on the OD. I have been thinking of clamping and sealing a plug using a standard vacuum SS tube fitting welded to a conflat for my -70KV Spellman DXM.
You need copper between the plug body and the SS surface for a vacuum seal. A 5/8" SS tube fitting would require the fabrication of a copper olive. Alternatively with a 3/4" SS tube fitting a thick walled copper pipe should work. I believe this eBay item 310578064543 is an electrical wire crushing splice connector, 18.86mm OD and 14.89mm ID, unfortunately it is slightly undersized for the spark plug so a bit of lathe work is needed.