Has anyone done something like this - any advice?

I think cole jaws would do it. Hold the inside and turn the outside, the hold the outside to turn the inside.

You can make them or buy them.

-- My reality check bounced...

I’ve turned crazier stuff than that screwed to a plywood faceplate
extension. You’re right about bolting the plywood to the faceplate.

Use old-fashioned wood screws to secure the work to the plywood –
they are tougher and less brittle than the modern black or gold
screws we use, which are sharp because they are hard, but brittle
in cross-loads.

I made something similar about 4 years ago Rich. My approach was to cooper or segment the ring as you are doing gluing all the segments together but I made mine twice as deep as it needed to be so that I could turn the outside shape as you would a bowl & shape the inside (like hollowing out a bowl) & then when I had it to a stage I was happy with after sealing with friction polish,I just parted it off & left the coopered offcut on the backer board a little hand sanding to the parted & a finish polish. Mine was only 6” Diameter X 1 1/2” deep & I turned the corners off rather than the bandsaw I thought it was easier for lining up purposes just to mount & centre it once.
Just my thoughts it worked for my project I hope it will be of help to you
Best
Trevor

-- God created wood that we may create. Trevor East Yorkshire UK

Back in the olden days of my youth (high school) we turned bowls that were about 12 inches in diameter. We used a face plate and used wood screws to attach a clean piece of pine board. Plywood would probably be better but we didn’t keep it in our shop. Then we would use a piece of thick paper off a catalog back and glue it to the paper then glue the part to be turned on the paper and clamp it. We used plain old fashioned white Elmers glue because that is what they made in those days. I NEVER say one come off the paper. When we finished we would put a chisel in between the bowl and the pine block and split the paper. I haven’t tried it in ….well many years but it did work. The better it is centered of course the more it would be balanced. Oak woud take a lot of power to cut. I think we mostly used African mahogany. Loren is correct about the black utility screw being brittle. Old fashioned steel wood screws are stronger. You could screw it to the plywood and put the holes down. We used the paper instead of screws so we could make the bottoms of the bowls thinner.

Rich, I go along with Trevor, just add a sacraficiial section to the bottom, could be any old scrap you have lying about the shop, sounds like your church has a good willing helper :)) (how’s the communion rail standing up?)

-- Pete G: If you always do what you always did you'll always get what you always got

I’m not a turning expert but Trevors approach sounds good.

-- https://www.artisticwoodstudio.com/videos wood crafting & woodworking classes

Thanks to all for the excellent input.

Just did the final glue-up and tomorrow is turning day. I’m looking forward to it and, thanks to all of you, I feel a little more confident about this project.

-- Rich, Cedar Rapids, IA - I'm a woodworker. I don't create beauty, I reveal it.

FWIW – The job is done and it went well.

Probably the hardest part was the glue up. I needed perfectly flat and flush gluing surfaces and when putting in the final piece I did not initially have flush surfaces and I had to run a piece through the joiner with the fence tilted just the right amount. By trial and error I got it.

I screwed a piece of plywood on the bottom and bolted the plywood to a face plate.

The turning was pretty easy.

Once it is stained and finished I will post the project.

-- Rich, Cedar Rapids, IA - I'm a woodworker. I don't create beauty, I reveal it.