//!OpenSCAD

difference() {
  circle(r=40);

  circle(r=35);
}
union(){
  square([5, 5], center=true);
  for (j = [0 : abs(30) : 360]) {
    rotate([0, 0, j]){
      translate([1, 1, 0]){
        square([4, 4], center=false);
      }
    }
  }

  for (i = [0 : abs(30) : 360]) {
    rotate([0, 0, i]){
      translate([18, 18, 0]){
        square([10, 10], center=true);
      }
    }
  }

}