I would never do that that, one will be enough, also if yo need to prop a ladder against the wall or do some work to the wall then the lanterns will be in the way.
Those eagle eyed amongst you will have noticed I used 50mm timbers for the upstand uprights so that I could add another 25mm to face of the internal uprights (as well as stuff between the uprights) so as to help with the thermal resistance.
get prices of sizes and decide from that. Perhaps they exist on shelf sizes as well as bespoke (like velux do) and are cheaper
For suppliers, I'd contact Architectural Products Limited and ask if they do them; they were great value for money for the roof windows I needed during my build
Also, if you're looking to save money, a lantern seems a bit crazy compared to a flat pane of glass, I think; they're more to make, a lot of engineering and have a higher surface area (lose more heat) than a flat sheet..
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