Need some geometry help

[Mjolinor]

OK, then you have to do it this way for circular ones.

rotate_extrude(convexity = 10, $fn=100)
translate([6, 0, 0])
polygon(points=[[0,0],[0,10],[10,10],[10,0]], paths=[[0,1,2,3]]);

The polygon defining your cross sectional shape.