I'm not sure about the roof supports possibly steal them from the Bajoran map, or other premade?
As to how I would make it enterable was to use a building just as the door, when you interact with the door the building vanishes and opens up the way to the 'interior' as you enter a reach marker activates and another building spawns filling up the gap left and closing the interior up.
The building will be interactable to leave again.
or alternative use invis objects to close the gap instead of waypoints.
You almost got everything right. The floor is indeed the internal of the of the U shaped large Vulcan building.
The ceiling woodwork is the Vulcan tower. Sink it into the ground ad place them 9m-s offset from each other. You have a great looking roof/celing structure.
The way to enter and leave is via triggering, unfortunately, but still, it is something.
To enter the inside of a building is relatively simple. If you can't move the player, move the building!
Have the player walk up to the door like really close. To do this, just place an invis object inside. Then have that interact hide the original building and spawn up a new one slightly off position so the playe is now inside.
The leave is the same. Move the player to the door, have an invis trigger spawn a building in the original position and hide the offset "entered" building.
This is one off deal due to the triggered nature, but can add nice immersion without map changes. Also if you use actual walls and not buildings on the sides, you can have functioning windows. If you look out, you see the village, people walking around and all. This, too, adds immersion.
This vulcan building is good for that. Also, there is one with a hole inside. That would make a great trap door entrance. Also the Fed conference center works very well, too. It is pretty bug and has 2 floors inside with floor and ceiling.