High-insulation 1:1 wideband transformer?
 

SpamHog wrote:

Now off to buy some CRT high voltage wire for the DC grounding / DC
block wideband transformer.

I have also been advised to look into NEON LIGHT WIRE. Working
voltages in the 7000-9000 V range, bearkup must be much higher.
Available in high current versions as well. For a big neon sign, 1A @
7000V is not unusual.

I suspect CRT wire is still better insulated, but neon is more easily
available.