The way is reduce the current density where it's higher and reduce electric field where it's higher.
Usually the connection between coax connector and cavity is a zone with high current density, use very thick wire.
The zones at High Zo are also interested to High |E| field. A thin sheet of dielectric (PTFE) increase the breakdown voltage.
Also don't use fireable materials such plastic.
Dissipate heat with a radiator.
The voltage breakdown usually occurs first. There are special gasses that are used to fill waveguides and cavities to increase the breakdown limit. Are you really running megawatts? I suspect that there is a sharp corner on part of the conductors.