$fn = $preview ? 40 : 360;

union(){
    translate([0,25,29/2+2.5])
        cube([20,50,5], center=true);
    translate([0,25,-(29/2+2.5)])
        cube([20,50,5], center=true);
    translate([0,-2.5,0])
        cube([20,5,39],center=true);
    translate([0,-20,-14.5])
    difference(){
        rotate([0,90,0])
        difference(){
            cylinder(h=20,r=20,center=true);
            cylinder(h=21,r=15,center=true);
        }
        translate([0,20.5,10.5])
            cube([21,41,21],center=true);
    }
}