The ceiling is made of custom-made GKL with hidden lighting.
The model is designed for smoothing.
To insert the built-in light sources, I recommend using the CoronaSlicerMtl material so as not to cut the geometry.

Made in Corona 9.
The model uses CoronaLegacyMtl materials, which may not be displayed correctly in very old versions of the engine.
For Corona render, it is recommended to install a version at least 1.5, since the glossiness of the materials is configured with PBR in mind.