The problem I have with hard mounting a rotisserie on a drum is that you will find it a bitch to deal with.
With a ring you can lift the whole setup off instead of standing over a flaming charcoal basket trying to wrestle that bird off the spit or trying to thread the rod with meats on and likely dumping them into the barrel. There is no way a Rib-O-Lator will be of any use in that setup.
It looks like you are cooking with an indirect setup, which is blasphemy IMHO. Let us know how it works out for you!
but thats just me