SSE Using only a few roombounds and portals in a cell

[itsfreerealestate]

I have a hallway that I want to use to load another room, but only when you enter that hallway. But doing this appears to force you to have to cover the entire cell with room markers and portals... which is a pain since the shapes overlap. I don't mind the entire rest of the cell loaded at all times, I only want the one room past that hallway to load in only if the player goes to that hallway. Is there a shortcut to do something like this or do I have to map the entire cell if I want to do this?