I've done these. I used the most conservative of a cantilever wall for the vertical reinforcing and and a pinned bottom circular wall for the horizontal. I wouldn't use the rectangular tank approach as the corners are missing.
Since you didn't ask, I would throw away the PCA Book on rectangular tanks (not the one on Circular ones) and just use "Moments and Reactions for Rectangular Tanks" by the US Department of the Interior, Bureau of Reclamation. It's been out of print for about 40 years, but I've found it legally online.