Hello, I have a weird problem, and not sure how to go about doing it. I need to make a part on a lathe, that has a no so weird feature, however I'm not sure how to go about making it w/o CNC.

I need to do this (image is a cross-cut of a tube, to show profile):
Click to view attachment

Right now I'm free handing it, and it's OK, but barely. I see radii tools for the lathe, but i'm not sure how to get it to do the inside radius. On CNC this would be some what simple. But I dont have a CNC lathe, so I need help finding a way to do it myself.

I was thinking about grinding my own tool, but if they already make a tool to do this, then why bother, right?

Anyone seen a tool to do this? I'm thinking about a 90' tool. Something that just does 90' worth of a circle, and I'd just repeat it 4 times.