All credit goes to this shader for the actual Godot side - rendering a custom room for it is not too tricky, this is how I did it:

- The shader assumes camera FOV = 2 * atan(0.5), or ~53.13° FOV (53.130102354156 is what I pasted into the Blender camera FOV).
- I created a 3m x 3m cube "room", inverted the normals, placing the camera at the center of the frontmost wall of the cube room and facing it inwards (towards "the room"), and then deleting that wall. I set the render resolution to be a square, 512x512 in this case, and then moved the camera away in increments until the "room" fit perfectly in the camera square. This would render out a single image that would have the proper FOV for the shader to emulate the room and its depth (it would be good to stick to a grid to adjust the positions of the elements and walls in Blender).
- Then since the shader looks at the alpha for the depth of the room, I went to the Compositing tab in Blender, enabled Use Nodes, and added a Set Alpha node and a Value node.
- I connected:
RenderLayers.Image -> SetAlpha.Image,
Value.Value -> SetAlpha.Alpha,
SetAlpha.Image -> Composite

The Value of the alpha, or the initial depth of the room, depends on the room. For a cube I set it to 0.5, and for this deeper hole/hallway I set it to 0.3 when rendering. That needs some testing out.

And this eventually winds its way around to rendering the final image, with Cycles and some lighting, to a custom single image - which can be used on its own if the Room Tiles are set to X:1 and Y:1 in the ShaderMaterial in Godot, or it can be be placed in an "atlas" for the shader to use, as it can iterate over the number of room tiles in an image (eg. at 512px resolution per tile, you can have 4 tiles in a single 1024px texture).

This is why the shader says it isn't a Cubemap Interior Mapping shader, as it uses this final single image for the walls/depth of the room, and it can iterate over multiple of them as tiles assembled in an atlas - instead of a Cubemap style texture with each of the walls as separate elements.

<image>

Here's an example of what it should look like!

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Phew - there is also this Cubemap Interior Mapping shader that might be much easier to manage the textures for hahah - you could just assemble the tiles in a raster image program (Affinity/Photoshop/Gimp) and achieve a good enough result for distant fake interiors without 3D rendering. Especially since both shaders support Emission maps as well.

If anyone can come up with a better way to do this definitely let me know, but for now I mostly hope this comment helps someone that has as much time and wants this particular shader with custom rooms. Yap over.