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You might talk with a local sheet metal worker (such as forms duct work at a heating and ac company) and see if they can build you a form for pouring a concrete pot.

If you are DIY inclined, you might check out the aluminum flashing at your local big box building supply store and form your own -- but be aware: that stuff can cut your hands if not handled carefully.

I have the same issue. Seems as though DIY is the best alternative. Any suggestions about what time of flexible plastic would work best? LOOK AT THIS SEARCH, https://www.youtube.com/results?search_query=DIY+donut+plANTER. People use anything as the basis of the form, pool noodles, tires, anything literally and then the glue stuff on the free form. Maybe I'll do it with a pool noodle and shells. I live in Florida. Would be appropriate.

Barbara Kanter ... Smooth non-porous surfaces will be more likely to produce smooth surfaces for your planter, inside and/or out, and less likely to stick to your concrete. You might also consider creating your wrap-around pot in two parts instead of three if you're going the DIY route.

I have three family members with 3-D printers....I might see what they can come up with. I am looking for something I can put plants in to form a circle around an 12"-15" opening for a small floating fountain (in a bowl).

I have three family members with 3-D printers....I might see what they can come up with. I am looking for something I can put plants in to form a circle around an 12"-15" opening for a small floating fountain (in a bowl).

For a diy approach, you could always try large wooden stakes (cedar?) or stones made for raised bed walls. A townhome I bought had a bunch of those laying around or maybe I got them at habitat for humanity’s store. Because the bricks are wedge shaped they curve the wall easily. You can see that in the right side. Home Depot had them when I searched for wedge garden wall bricks.
Black pond liners are another possibility.
Amazon has 4 ft galvanized round raised beds. If you can used a large pot with the center cut out to protect what is in the middle thats a solution. Or use the raised bed or pond liner as a casting solution for concrete.