I seen some details on the GTBA site once on making a spot welder that might help, also 304 spec stainless is ok 316 would be better.

Got the guys in my workshop to make one once for a 14lb turbine, made the inner diameter the same as the NGV with the front flared out, length was just made to suit model.
You make the zig zag spacer between the walls with a strip about 10mm wide and fed through a gear mesh, or just fold with pliers..

Spot welder is important and makes the job fairly easy, check out GTBA or google on making one.

no doubt someone will correct me and give a more accurate method of calc dia etc.

Gap from tailcone to tube is important, 15-20mm seem to be the norm