coop you could make the expansion chamber out of carbon fiber cloth and hi temp epoxy
10 years and 30 pounds ago i race superbikes and one of the best exaust systems out there was made entirely of carbon fiber

you could look at some tuned pipes that you like then sand a piece of foam to the shape you want then laminate with CF and high temp epoxy .when finished scrape out all the foam guys on the scratch building forum could explain working with composits very well