How to light a hollow BSP?

I currently have a hollow BSP which eventually when I am done editing it will be converted to a static mesh, but even when I convert it to a static mesh to test it, it won’t get lit.

Yes, I put lightmap coordinate index on 1 and changed the LOD, but that isn’t the problem.

The room is really big and I am looking for a way to make it lit with the least amount of lights.

A sky light or a directional light are not an option due to lighting the whole level, while I only want that box to be lit.

Is this possible with a post processing effect? Or how do I do this?

A screenshot of the room:

https://i.gyazo.com/92098f059d3f2c4998b8fcbbd3da9fc3.png

Size:
15000x10000x5000 hollowed out, the walls are 40 units thick.

You can indeed use a post process effect: in auto exposure you can increase your exposure bias (You need a post process volume in your scene, then go to settings > Auto exposure > Exposure Bias), take in mind 1 is 2 times brighter, 2 is 4 times, and so on.

You could also try placing pointlights, disable the shadows if you dont want them. If using static lightning increase the indirect light value of thos lights; that will make the bouncing light more intense. Rebuild your light afterwards. You will need quite some lights however, that room is huge (150 mts x 100mts x 50 mts, thats bigger than the average football stadium).

I tried it, but this just made the lights really bright that are on the objects inside the box, what I want is a light that lights the box, not makes every light brighter.

well, there is no easy answer here. Do you have a picture of something like what you want to achieve?.