oring_diameter = 53;   //-- Interior diameter
oring_width=3;

difference() {
  rotate_extrude(convexity = 10, $fn=50)
    translate([oring_diameter/2 + oring_width/2, 0, 0])
      circle(r = oring_width/2, $fn = 50);

    translate([0,-oring_diameter/4-oring_width],0)
      cube(size=[oring_diameter+2*oring_width+2, oring_diameter/2+2*oring_width, oring_width+2],center=true);
}