As of GEK v2.3, we've upgraded all kits to include a stainless stell reduction bell. We're making these out of 16awg 304 sheet steel, rolled and welded into the desired cone shape. We were previously using a cast iron redution bell, cut from a found 3" to 4" plumbing increaser. While this worked reasonably, we could not vary the dimensions, and thus have wanted to move to a purpose made bell that we can make in any dimension. Here's what we have now. The default 3"smD x 6" lgD x 6" tall bell is on the left. Other ratio variations are shown to the right.
Making a rolled stainless reduction bell
You can make any of the above 3" restriction bell versions using the cut files below. Later i'll also post these for other restriction sizes, but currently the files only cover 3" restrictions. You can trace and cut these shapes easily with plasma cutter, torch or sawzall. (Remember to wear a mask with stainless) Then roll the cut sheet into a cylinder with a slip roller. Or bang them out with a soft sledge around an appropriately sized forming tube. Of course welding them together with a stainless welding set up is best, but you can also do it with mild steel rigs. The weld may detoriate over time, but you are likely to change your mind on design particulars way before that happens.
Full res versions of the cut files are available in .pdf and .dxf at the links below.
3InchReductionBellVariations.pdf
3InchReductionBellVariations.dxf
These cones weld to a ring 7.5" in diameter, with an inner diameter of either 6" or 4.5", depending on the cone. I forgot to include these in the drawing. Will fix later. You can make the ring base out of mild steel and weld it to the stainless. The base of the bell is not in an oxidizing atmosphere, so mild steel is fine.
These cones should be used with a tar fence around them. Tar fences are explained here.