You are correct, hence I suggested a gasket of some sort (copper washer?) that’s hopefully wide enough to seal up. Just don’t know how well that will do under pressure in the long run
I mean an o-ring should suffice as long as there is a smooth/chamfered surface for to to run on for both the cooler and adapter side, 100psi is nothing on an o-ring these days (my built motor truck runs 80-85psi cold on 20w50).