In my mind's eye I'm seeing something resembling a large Lazy Susan style plate that is driven from a vertically mounted stepper motor via a timing belt from off to one side of the rotating plate. The whole affair is assembled on a single rectangle or square base plate that is removable from the CNC machine table top. The rotating plate could have clamps of some sort on it that allows mounting various size drum shells. It would need to be designed to be as low to the table as practical. If the rotating plate is made on the CNC machine it could have teeth around the edge of it (like clock gears) and the teeth would need to closely match the timing belt teeth.
The motor could also be mounted so that the timing belt pulley faces downward and is aligned with another timing pulley that is under the rotating table.
I think it is possible to build one to do this job, but it will be some work to get the drum shells centered and well supported for the drilling operations.
You may want to build some trial mechanisms from plywood or MDF and the largest Lazy Susan base you can find that will do the job. Some sort of home brew radially mounted skate bearing support may be less wobbly.