Ideas to fill up the space?

[PutraenusAlivius]

- A large window: Architecturally, I cannot fit one in on either end of the room, and wherever I can, it would either be too high or not large enough.
- A "hole" in the roof: There is a storey above; which is actually essential for the story.

Make the scales the same. Turn off grid snapping if it's too high/too low. When you get the exact size, make sure the z and the x axis are the same as the y axis. If the walls surround it doesn't fit, add a pillar in between.

If the room above the ceiling is empty or serves no purpose, make just the door and make it locked. Now add a hole.