Doug, I really cannot remember, was made from bits of scrap from the farm workshop, Will see if I got more to measure.
Thing is, you can use cast iron if you want cheap but will be heavy, Inconel is good if you want lightest, but you better be a good welder. You make the choice.
My first setup was made from 3mm mild steel (again scrap bits of cattleyard rails) worked fine for a bit but design was loud and became obvious in that it was not going to last longterm.
Best to just make something to get flying. Not really much effort in upgrading to better materials later, failures tend to happen over time so If you inspect after each flight any cracks/deformation that appear will be obvious long before failure, main thing is to expand the exhaust gas as soon as possible as that drops temps fastest and get it out of the cowl with the least amount of pipework. There are tricks to quiet down a loud system that can be retrofitted later if need be ( similar idea as a rifle silencer) to quieten.
Andrew