Suggest square stock, split length-wise then routing each piece with a "half round (cove?) bit, then glue up the two pieces (using a waxed
1/2" O.D. dowel or aluminum rod/tube to align, perhaps), chuck in the lathe and round to desired O.D.After the glue up, you might be able to run a 1/2" bit down the center (without "blowing out the sides") to clean up the hole.
Caveat - I have never tried anything close!