The rotisserie ring that fits on our friend Airfoils grill and all other 22.5: Weber grill is available directly from
Search for the Weber store.
I think most would agree that building rotiserrie ring for a square style smoker would most difficult.
I'm not even sure a usable one could be fabricated for a round or oval one. A-foils has valid points regarding the join areas being out of round, and the ears to keep it in place.
However I have a sheet metal fabrication shop down the street from me that specializes in making aircraft frames and the curved sheets of metal that form the skin of the plane.
They just need the diameter of the finished ring and they say they can match it close enough to be usable.
Let you know of the success or failure and also the cost if it works.