non boxed shape of triggers? How to deal with those? Trigger "grouping"?

Hi all this might be a bit silly question, but Im dealing with situaltion here when I have a house and running different animations based on when PC enters/leaves each room (OnTriggerEnter/OnTriggerExit). It works perfectly well for box shaped rooms, but I have also one room that is NOT box shaped. So I've tried several approaches here: 1/ use mesh and mesh collider as trigger. Unfortunately, I have to select "Convex" before making it "Is Trigger". And when I make it convex, it changes the shape the way I don't want. So this is not a solution for me. 2/ I've tried to add two box colliders within one object. It does not work neither. Is there any way to "group" two colliders and make it behaving as single entity for OnTriggerEnter/Exit?

see attached screenshots.

Thanks for help in advance Milan