I've done them for my 14.5 & 18.5 WSMs. It's some work, but you can save some cash. You need to weld or have a friend that does. I made my own rings out of 16 gauge metal. If you don't have a ring roll, try making a form out of wood.to bend the metal to shape. Clamp the metal to the form & weld it together. Weld some tabs about 3/4" from the bottom of the ring so it can slip into the bowl a bit (that's Weber style). Get some 1/4" rod & bend it around the top of the ring & weld it. This will allow the lid of the kettle to fit snug. Now all you have to do is cut a slot & drill some holes for the bracket & spit bushing. Paint the outside with HH Rustoleum. Onegrill is a good source for a bracket, spit, motors & etc. Good luck.
fwiw - I bought the Cajun Bandit rotisserie setup for my 26er. Getting lazy, I guess.
This is one I did for the 14.5.
Turn it upside down & it would fit a SJ.