Here you can see that the 3D working model has the dimensional specifications of each individual piece of the pipe drawn on it, this makes the job of creating individual paper templates easy for cutting the sheet steel. You can see the metal pieces for the front sections already cut out.
For the best results i would recommend 2x 2 meter square sheets of steel of only 0.7mm thickness (1 sheet per expansion chamber), although you can use stainless steel which is harder to shape and also splits more easily.
Please note that when your planning out your pieces that you want to cut, make sure that you wear a good pair of safety gloves as the edges can be very sharp! all you need is a good pair of Tin snips and a couple of rolls of Scotch tape to hold the paper templates in place while you cut out the steel pieces.