My 1:1 high-voltage transformer is coming along well. This is to make 5V, 12V, or whatever at high voltages. I'll be using it at 6kV, but it's passing tests at 9kV, and I'll do a weekend at 11kV before pronouncing it OK.
The transformer is a small 50-cent common-mode choke, TAIYO YUDEN p/n TLF9UA102W0R8K1, footprint only 11x17mm. The pins are spaced 8mm. We potted it with vacuum air removal, Dow Corning Sylgard-184, otherwise called PDMS. Testing with +5kV and -4kV, to avoid corona discharge.