difference(){
union ()
{
cube([20,45,31]);
// translate ([15,0,0]) cube([10,45,6]);
}
union()
{
translate ([-0.2,22.5,20.2]) rotate ([0,90,0]) cylinder (h=21,r=14.2,$fn=48);
translate ([-0.2,4.8,20.2]) rotate ([0,90,0]) cylinder (h=21,r=2.3,$fn=48);
translate ([-0.2,40.2,20.2]) rotate ([0,90,0]) cylinder (h=21,r=2.3,$fn=48);
}
}