difference(){
    hull(){
cube([10,30,40]);
cube([30,30,10]);
    }
translate([5,15,-1])cylinder(h=45, r=2, $fn=20);
    translate([-1,5,5])rotate([0,90,0])cylinder(h=45, r=3, $fn=20);
       translate([-1,25,5])rotate([0,90,0])cylinder(h=45, r=3, $fn=20);
}