Flyback Questions

[Electronic M]

You may want to look at some of the early Solid state transformers....Those used an external recitfier box. Those boxes usually contained a doubler, tripler, or quadrupler circuit. If you find one for a doubler or tripler you may be able to get enough HV from one of those...They usually over built those flys such that they could run near 2-4X their ratings and live*.
Another possibility would be to forcibly short the internal diodes on newer flys. It may be possible to do it internally by taking one apart, or you could hit it with more than the designed voltage for a while. IIRC lightning will cause power supply diodes to short in the main PS stages of TVs. So maybe a brief jolt of ~100KV will short the internal HV rectifiers in a modern flyback.

Have you considered a tesla coil? IIRC those are AC output and easy to make out of easily obtained materials.

* For example early solid state Zenith sets had problems with capacitors failing in the drive circuits and causing the HV to shoot up to ~60-100KV The voltage would arc through the CRT glass and cause the glass to crack and the CRT to fail.