Interesting! So you think it is possible to fit that size into the available 1.3m passageway. I'd long picture the roof as being pitched with two large "velux" to maximise light and critically maintenance access through them. I've look on various sites (theirs, outlets, etc) but none seem to deal with the unusual spaces.
Looks like that option is back on the table. The only one that seems to have received no votes is s flat-roof with some form of roof light - oh and those "futuristic" looking light dome things!
If you can build some framework in your passage way to except the frame of a Velux window then once you have fitted the frame you can then flash the window from inside the aperture. If say the lobby is 1.3 wide and you used one of the SK range which is 114cm wide then a piece of timber say 6x2" each side brings you to approx 124cm leaving 6cm to play with.
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