Using SIW slot coupling junction to pass RF signal between two PCBs.

Assume the first PCB is microwave oscillator, and the second PCB is some microstrip circuit (antennas, mixers, counters, etc.). Both PCBs can contain SIW with slot, and when placed one on top of another can be tighten with screws. I am searching for most simple SIW slot configuration can be used here?

SIW means?

well, obviously if you have an SIW transmission line, it has a wave impedance. I would make the "slot" be of waveguide dimensions such that it has the same impedance, and is not so narrow as to be "cutoff".

I would also put a conductive elastomeric gasket around the slot, so that when both boards are held together, there is an excellent ground plane to ground plane contact.