The best stainless exhaust
riser fabricator is located in Ft. Lauderdale: http://www.deangelomarine.com/
They make exhaust risers and systems for the mega-yachts. I had one built for my Perkins
6.354 diesel back in the early 1990's and it is still in perfect condition. You either need to give them an example or some engineering drawings of what you need. They are not cheap
but the product lasts longer than you ever could imagine. After running for 12 hours at full cruise
I can put my hand on the top of the riser and it is only warm.