Update:
When I first ordered the 21st Century Neck Turning tool, I ordered the tool and the head stock support. I was planning on running a drill and thought it would be fine. After turning the first 100 cases I ordered the tail stock and one of Pete's shell holders. I didn't order the base to make a full lathe as I have been learning Fusion 360 and a friend of mine has a Tormach that with careful supervision is allowing me to test some of my designs. So I decided to make my own base.
When talking with Pete, he indicated that he had a new case holder that was more rigid than the current model. I had ordered the Flexible model and Pete offered to send me one of his new Rigid models to try out. I must admit, both are built to the standards one would expect from 21st Century. I did 70 of the next batch of cases with the Rigid case holder and was amazed at the control and quality of finish that I got.
On the next 5 units, I switched to the Flex case holder and must admit, I didn't care for the flex and found it more effort to get the case on the mandrel than with the Rigid case holder. I pulled the Neck Turning tool off of the head stock and holding the Neck Turner in my hand used the Flex case holder on the last 25 cases and found that in this manner, the Flex was much better at allowance of any misalignment.
So my conclusion, the Rigid Case Holder is, IMHO, the only way to go with the lathe configuration. There is more than sufficient flex in the Neck Turner mount to compensate in the lathe config. If you are going to hold the Neck Turner in your hand and drive the case with a drill, then the Flex Case Holder is the right approach.
I want to thank Pete for all his advice, letting me try his new Rigid Case Holder, which will be available as of 9/1/2022 if my memory serves me correctly. I am very happy with my choice of supplier.
P.S. I went back and did the last 30 cases again on the lathe. So much nicer finish.
Here is a image of one of the cases, sorry my phone wouldn't focus directly on the case:
View attachment 1364209
Here is a video I took of one of the cases being turned. I was running the drill in my right hand and holding onto the phone in my left. Sorry for the movement. I ended up turning the drill away from me and letting it rest on the bench. Then I could apply a little force in or out and the vibration of the drill helped me move it in a fairly controlled fashion.