Here is a Hi-Pot tester I have just finished.
Hi-pot tester is good for 11Kv at the terminals and the leakage current is 3uA.
I’m pretty happy with the result.

Testing a Burle 8792 transmitting tube. Leakage is 3uA @ 7Kv

Inside view from the top

Schematics

EDIT: Transformer listed in shematics is not correct. You must use an linear type. The one listed is electronic and no good for this use.