function hole() = 8;

difference() {
    cylinder(20,10, 10, false);
linear_extrude (25, true){
    square(10, true);
}
}
difference() {
translate ([0,0,20]) {
    cylinder(20,10,10, false);
}
translate ([0,0,20]) {cylinder(25,hole(),hole(),false);
}
}