Probably any will work, but wear out your ring faster and cause rust at the entry points if the square section of the rod grinds on it.
For example, Weber gasser roti kits can often be fairly inexpensive. They have a longer spit rod (which by itself isn't a concern) but only ONE of the two rod "indentions" (rounded sections) will line up. The other end will have the square section slowly grinding against the ring edge.
The charcoal kettle spit rod is 28.5" length, not including handle. The "indentions" occur of course at about 22" apart, so aim for that regardless of spit length. The Weber gasser's spit indentions occur at least 23" apart, or 1 1/4" too much when attempted with the kettle ring.
I know it's not a big deal to some people to match round spit rod sections with the grill, but as you're probably aware the kettle roti attachment is the most expensive aspect of that whole kit, so it's best to not prematurely damage it with the wrong spit rod.