$fn=25;

module holder(){
    difference(){
        intersection(){
            translate([10,0,0])cube([40,20,3],center=true);
            translate([20-7,0,0])#cylinder(r=20,h=3+0.1,center=true,$fn=150);
        }
        cylinder(d2=6,d1=5.5,h=3+0.1,center=true);
    }
    translate([30-1.5,0,10-1.5]){
        rotate([0,90,0]){
            difference(){
                translate([0,0,0])cube([20,20,3],center=true);
                cylinder(d1=6,d2=5.5,h=3+0.1,center=true);
            }
        }
    }
}

holder();