inner_dia = 17;
thickness = 1.6;
outer_dia = inner_dia + 2*thickness;

module ring(){
    translate([0,-outer_dia/2,0])
    linear_extrude(height=2)
    difference(){
        union(){
            circle(r=inner_dia/2+thickness);
            translate([-outer_dia/2,0,0])
            square(size=[outer_dia, outer_dia/2], center=false);
        }
        circle(r=inner_dia/2);

        squ_height = 15;
        translate([0,-squ_height/2,0])
        square(size=[inner_dia-thickness, squ_height], center=true);
    }
}

union(){
    ring();
    translate([0,thickness/2,0])
    cube(size=[outer_dia, thickness, outer_dia], center=true);
}