Since the Rib-O-Lator, nor the EZ Que cradle would fit on an 18.5 inch WSM, it would seem to be more of an issue of practicality to me.
If you just drilled a couple of holes to mount the brackets and the spit rod, I would think that most any generic rotisserie could be used.
I just don't see anyone making a separate ring as the demand may not justify the costs. Just my opinions.