Under normal circumstances you would not do this: 70mm diameter, 120mm stickout off the chuck, especially with plastic this can go very bad very qick.
There is a high probability that the part works itself out of the jaws, even with a six jaw chuck.
Robin Renzetti showed a very good trick on Instagram, how to hold something like this very secure.
All the magic is a tiny lip turned into the material that engages one of the reliefcuts in the jaws of the chuck:
That setup allowed me to turn this PVC part in one setup on the OD, ID and face, and then to part it off.
That lip helps also to save on material. This delrin part is clamped on 3mm of stock:
To turn the clamping lip I use a DCMT insert, the backside of the insert creates a chamfer that usually fits the relief in the jaws quite well. Make the Lip slightly wider than the relief in the jaws so some material is deformed during clamping and creates a very solid clamping situation.
To turn the lip on very long overhanging material, you have of course to be very carefull or use, if possible, tailstock support.