For a 1 off at that size I would make a 60 degree point on a piece of silver steel, then file away the “top” to make a d bit. Add a little clearance on the none cutting bits. Harden and temper to straw.
To use you could hold in the tailstock and drill, or use like a boring bar in the toolpost.
for a larger cone then setting the cross slide over at 30 degrees (half angle) and feed in with that to bore – which is probably what is meant. The problem with that is the bottom of the bore – if required to be a point will need a boring tool which is very pointy.
Dave