My interior cell is actually pretty much done. I’ve been NavMeshing the thing too, and this is how it looks in the editor when I’m doing that job. you shouldn’t forget to “Finish the NavMesh” once done, so that NPC’s can enter and exit the cell. It’s actually very important; you just need to have your door marker standing on one of the triangles and press the V on the NavMesh toolbar to finish it properly.
A cell is actually never really finished. Knowing myself, I will probably move things around and add stuff as I go, and it will only get better. Now to add some new NPC’s to the interior and set bed ownerships and stuff like that.