Plenty of air clearance to withstand many kV.
If you prefer, you can buy shoulder washers that fit snugly over the thread, centering it within the hole. You will need one for each side of the panel, the panel will have to be as thick as the shoulders are tall, and if you truly need a large clearance distance, you'll need one more piece of tube that fits snugly over the thread (inside the shoulder washers), or over the shoulder washers, or both. And some nice and goopy vacuum grease or silicone (non acid curing) to pack the hardware with, to eliminate most air space.
With that, you'd be able to do over 10kV, which is more than the post-to-post gap will take. At that point, you should seriously consider HV type connectors with very deep insulating shrouds...
Tim