Select the vertices highlighted below. Chamfer them 9 units.
Go to the Rendering rollout of our shape. Check both Enable in Renderer and Enable in Viewport, set the section to Radial with a Thickness of 14 and 8 Sides. Right-click and convert to an Editable Poly or throw an Edit Poly modifier.
Go to Edge sub-object mode, select the two edges at the bottom that mess up our geometry and remove them (hit Backspace). Now select the edges that define around the bottom polygons and do a 0.01 chamfer.
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