As a side comment: I now understand why it is advised to use a mask when you sand or drill in MDF. After the first two holes I realised that I didn't want to inhale the dust. I don't have a mask, but when I was in Madagascar last year we were taken to a marquetry workshop where they used airplane eyepatches instead of mask. Think how clever people can be! And it works just great.
Anyway, I drilled all these holes. When I earlier had tested drilling in MDF with a large drill, the surface was damaged. So I first drilled very small holes with a small drill and then enlarged them with a power tool. Before drilling, I tested the position of the lights on a piece of cardboard. Then I traced the holes from the cardboard onto the roof.
I need larger holes than for converntional wires because I will push bulbs through them, like this:
I will later protect wires with heavy-duty tape and possibly make grooves because the main house will be on top of this. The battery holder will be on the back. But there are still some things I need to finish before I glue on the roof.
It is almost impossible to take pictures from the sides, through the windows, but at least ir provides some idea. One set of lights is under the staircase in the rear corridor where I have a chest of drawers. It is visible through the working kitchen window.
From the other side, you can see the staircase and the wall with some pictures.
Again, it is taken through a window. Finally, from the front entrance, you can see the staircase through a door.
I haven't bothered to make lampshades because the lights are quite dull as they are. When I have proper lights in the front rooms the corridor lights will be almost invisible, but they will still contribute to the effect. My filmmaker son tells me that in light technicians' jargon they are called "practical".
I am working on the proper lighting now and will hopefully be able to share the results soon.