Heres how to make usable feddthroughs out of spark plugs good to around 20kV with out any further measures
photo 1 choose a non resistor plug ie no resistor in the centre leg with as long a reach as possible (plug thread length)
Now you have to part off the bottom part of the threaded portion so that more of the centre is exposed
careful you don't want to hit the ceramic insulator inside.
using a thin diamond coated disc in a dremel cut the insulator almost all the way through all the way around
Remove sparkplug from lathe and holding the exposed inner in a vice gently bend the inner a little, the ceramic will crack where you sawed it all around with the dremel
things should look like this
Now the tricky part you will have to make a custom lathe tool to face an O-ring groove into the shoulder on the sparkplug.
Its not that easy as there is not much room and the toll tip is quite small and is easy to break. But at least the plug is made of steel not stainless which would make this operation nigh on impossible
The finished beast should look a bit like this