main_inner_dia = 60;
main_outer_dia = 70;
inner_dia_small = 40;
height = 15;

thickness = (main_outer_dia - main_inner_dia)/2;

inner_radius = 2;
outer_radius = inner_radius + thickness;

extra_size = (main_inner_dia - inner_dia_small) / 2 - inner_radius;

module ring(inner_radius, outer_radius){
    difference(){
        circle(r=outer_radius);
        circle(r=inner_radius);
    }
}

module quarter_ring(inner_r, outer_r,){
    difference(){
        ring(inner_r, outer_r);
        difference(){
            square(size=[2*outer_r, 2*outer_r], center=true);
            translate([0,-outer_r,0])
            square(size=[outer_r, outer_r], center=false);
        }
    }
}

rotate_extrude(convexity=10, $fn=100) {
    translate([main_inner_dia/2,0,0]){
        union(){
            square(size=[thickness, height]);
            translate([-inner_radius,0,0])
            quarter_ring(inner_radius, outer_radius);
            translate([-extra_size-inner_radius,-thickness-inner_radius,0])
            square(size=[extra_size, thickness]);
        }
    }
}