Jet Tech has made over 600 custom exhaust tubes since 1999.  Jet Tech LLC exhaust tubes are double wall construction made to your required length and outside diameters to fit your applications.  The inner tube dimension as well as the bell mouth size are determined by your turbine output.  The inner "hot" tube is .007 full hard tempered 302/304 stainless steel with a machined bell mouth.  The outer "cooler" tube is .005 full hard tempered 302 stainless steel.  Typical mounting is with 2 tabs at the 3 and 9 O'clock positions and the supplied aluminum straps with #6-32 screws. Exhaust Tube lengths can vary from a few inches to the maximum length of 54".  The tube shown in the photo is for the Ziroli Panther with a 22# thrust turbine measuring 19" long.  Prices vary by size starting at $195.   Contact Jet Tech LLC via email at jettechusa@frontiernet.net  for a quote and availability of your exhaust tube.