As I said earlier, the spacing of LED housings on the false roof does not match that of the compartments which is precisely 26.75 mm. Hence, how are these illuminated? It is theoretically thanks to a translucent diffuser equipped with a multitude of small lenses, glued under the false roof.
I did not have an a priori about this system. So, I made a strip respecting the pitch of the roof. Is it because of the LEDs different from those provided by LSM? Still, the lighting was particularly poor, and very badly distributed. So, I decided to make a more classic lighting respecting the pitch of the compartments. But this necessitated, on the one hand, to suppress the diffuser, which I obtained by swinging it from left to right with the aid of flat pliers to break its fixing pins one after the other, and, on the other hand, to drill holes into the roof, at the compartments pitch.
Rather than a drill, I preferred using a Ø 6 mill bit (the minimum drilling diameter for PLCC LEDs is 4.5). In my opinion, the milling gives a neater cut. The body is fixed into the vice of the milling machine, with a 10 mm thick, 30 mm wide plywood shim inside, and protected externally by a non-woven wiper folded to form a thick enough mattress — there is no significant relief likely to be damaged. There is no need to tighten the vice excessively, the machining forces being only perpendicular. On the diagram, the origin (the zero) is the middle of the coach. For the dimensions to be respected, see the PDF document.
In addition to these drillings, the various reliefs of the false roof must be levelled too.
In front of the drilled body (arrows), the lighting strip and, in the foreground, the suppressed light diffuser. Hover the picture to see more details.