When I was a kid I used to work in a machine shop. The owner used to make kettle "extensions" so he could fit 30+ lb. turkeys on his webers. I took his design, made it a little taller to fit 2 grates and created a middle section for a homemade 22 1/2 inch WSM. My dad and I have made 4 of these middle sections. I cook eight racks at a time on mine. Lots of fun! I even put mine on an old 3 vent curbside rescue weber kettle.
The way metal prices are now, one might just want to consider the Cajun Bandit kettle conversion kit and be done with it. I think metal prices are at least 4 times higher than when dad and I built ours.