Radius cutters are not that hard to make. But the larger they are the more they chatter, at least on smaller lathes.
if you want to make a radius cutter dykem the tool, draw your radius on it, and grind away. On a big radius I have used an angle grinder with a cut off wheel and slit it short of the line over and over. then I have removed the waste with the cut off.
Then I take points and using an air grinder use the point to final shape it. I don't like using the corner of my 6 or 8 inch bench grinders.
pink or white points are soft enough to grind away.
But a ball cutter would be a cake walk. I have used my compound loosened (swivel) and cut balls that way before I got a boring head. it worked well, just go slow.