You can make circles in the editor by placing 3 area or way nodes on its circumfrance, then pressing o if I'm correct. Then you can select and delete the nodes you don't need, by shift, click and drag then pressing the delete key. This should leave you with a smooth curve for a feature like this. To finnish off add the straight lines and hopefuly if you press q to make them square,that wont mess everything up.
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            Published using OSMCha: https://osmcha.org/changesets/152918142
            