Car body shop.... thin material......... Especially if they're used to fixing the last of the Rover cars that seem to be made from bean can material! To be honest, the No5 scabbards are not particularly light gauge. We used to dolly the dings and dents out on a mandrill
I am thinking tig weld because you get a strong butt joint but not so with silver solder unless it's an overlapped joint. There will be specialist welders/fabricators on a little industrial estate near you!
OR....... just unbraze the button and fabricate an extension to form the longer top section. Tig that on, braze button back on, drill mouthpiece retaining screw hole and that's it. That's what I'd do. Cheap or what?
Added a bit later.... If these long scabbards aren't easy to obtain, what about doing a short pictorial/photographic essay about how you went about converting a shortie to a long one. I'm sure it'd be useful. Just like the Mk5 Sten gun butt from a No4! Mind you, from the feedback, I think that was a bit beyond the ability of the average forumer!