Found a 6" cast iron collar for a back-exiting barrel stove flue.

https://www.amazon.com/US-Stove-4029.../dp/B004V7F99A

I built a steel barrel stove this year using the typical stove kit that's available everywhere. When it arrived, I realized the cast collar was curved of course, for mounting a vertical flue pipe on the curved section of the barrel. Needing to come out the back (barrel bottom,) I looked for a flat 6" one.

The one in the link above was the only one I could find online. It is a replacement part for a manufactured stove and the back of the collar was perfectly flat except for two small locator pins cast into it. An angle grinder took them off in a few moments. Traced the opening using the collar as a template, cut the opening with a saber saw, drilled the bolt holes and done.
(I've seen builds here where the end of a flue pipe elbow was cut into a series of tabs that got screwed into the barrel around the opening and was going that route had I not found the cast one.

Learned everything I know about building a barrel evaporator here so thought I'd share this in case someone else scours these posts for info as I was.

Wayne