I am sure this is not a new idea but I cant find one on the forum.
A simple turning job done at one setting (a decent part off will suffice) for this purpose, obviously for live centres only.
A big help when setting up in the 4 jaw to have something that stays in place while adjustments are made. Its main purpose however is to provide extra security when taking heavy cuts on a part on which you have less than an ideal secure grip.
Like this perhaps.
Or maybe you do not want to centre drill for some reason or perhaps a second op in soft jaws.
When toolmaking I made one for support for turning heavy die housings up to 18 inches dia, I witnessed one of these get away and a 1cwt of metal bowling along the shop is a sobering sight. You know you have got something right when the other turners copy it and its use becomes the norm.