Then select it all and “Intersect With Selection” and erase the unwanted geometry.
Now thicken the seat with the Joint Push Pull plugin. Thicken it by ¾” like we did the seat. You might get a couple of reversed faces so take some time to do any cleanup necessary. If you were doing less polygons on the arcs there would be less to fix but the model would not be as smooth. Then soften/smooth the faces.
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