Don't know if this will help, but there is an aftermarket manufaturer that sells double walled exhaust pipes for Harley Davidsons (to prevent the blueing seen on many chromed motorcycle exhaust pipes). Shouldn't be hard to find them with a search. Maybe they can help or lead you to someone who...
Minimalist, the problem with engineers is they are expected to know everything. Not wanting to let anyone onto the fact that they don't, they like to lash out at anyone who dares to ask a question. If they don't know the answer, they lash out with proportionately more zeal. How dare you ask a...