Put a center punch mark in the center, and then just hold it against your chuck jaws with the tailstock center.
It’s called pressure turning. If you take light cuts, you should be fine. But I would keep the RPMs on the lower side.
Alternative, Chuck up another piece roughy the same size and just out from the Chuck jaws and use this as the backing plate to hold the flame cut piece to.
It helps to have a live center.
If you do not want to center punch the plate, you can sandwich it between another piece that does have the center punch/ center drill puck.
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